1) Ehh, kind of.
2) Not just unlocked, but compatible with the phone network you'll be using. If using AT&T based networks in the US, AT&T voice and data bands will be needed, if T-Mobile then the same, if Verizon you'll need Verizon and CDMA band support. Typically, phone models fall into two categories here in the US: GSM, which covers AT&T or T-Mobile, and CDMA, which covers Verizon and the remaining husk of Sprint that's being re-farmed into T-Mobile spectrum.
You'll also need to make sure VoLTE (voice over LTE data) support is present for whatever phone network you'll be using. AT&T uses IMEI whitelists, so the phone has to be on AT&T's approved list for forward calling compatibility past February of next year. T-Mobile's VoLTE implementation is quirky and non-standard, so you'll have to shop likewise for pre-approved. If you're looking at Verizon, their VoLTE implementation seems to just work provided you get a supported CDMA model.
3) Swappa is pretty decent for used marketplace trying to CYA, and Ebay can be safe if you deal with either sellers who appear to be professional phone refurbishers/electronics recyclers, or obvious private sellers with good transaction histories who take care of their devices so well that they sell the stuff with the box and all peripherals. Buying used, you will want to do an IMEI check, to make sure the device hasn't been reported stolen.
Swappa has a universal IMEI checker, but it also wouldn't hurt to do an IMEI check with the phone network you'll be using the phone on.
Regarding specific models, I'll save that for later, and it's partially dependent upon what phone network you're currently on.
4)
-\_(ツ)_/
-Anyway, some additional thoughts and recommendations.
Network Thoughts: Not sure where you're from, but I'm gonna assume the Untidy States... if you're from Canukistan or elsewhere, YMMV. Although Verizon swore they were shutting down their legacy CDMA network last year, they postponed the shutdown indefinitely, however, they will not activate new handsets that don't have VoLTE support for their network. AT&T is on slate for doing the same with their 3G GSM network next year, shutting it down entirely, I mean. However, with T-Mobile already completely turning off their entire 2G/3G networks (outside of a small handful of 3G UMTS areas), it's possible AT&T's 3G network might get a stay of execution next year as well if for no other reason than international roaming agreements, but this is also the network that killed off their 2G network ruthlessly a few years back and this is the country that loves and pioneered planned obsolescence. If you want to play chicken with AT&T and phone voice support with non-VoLTE handsets past the next year, this is an interesting point to keep in mind.
Security Thoughts: If you want to play chicken with voice support on AT&T's network whether you're there already or willing to switch to, and given you're mostly wanting phone and camera, security issues aren't as high priority, excuse the Dropbox/Box thing (though it can be worked around if you don't mind still using USB transfer cables). This opens up some really cheap options with really nice cameras, like the Lumia 1020 at around $80, which is
still considered one of the best phone cameras ever produced in the past decade, but there's no VoLTE support, and Windows Phone as a platform is well and dead without security updates. You could do a forced upgrade from WP8 to WP10 to add on Dropbox and OneDrive support, but the camera software and battery life are better on WP8, and there's no guarantee that OneDrive and Dropbox will keep working on WP10 past next year or the next major encryption/security breach that changes basic login infrastructure.
Coverage Thoughts: Typically, it's best to buy a phone for the network you're already on. It's the corollary advice I give to people who are switching from postpaid on the major networks to prepaid on an MVNO, which is choose an MVNO on the network you're already on, as coverage should stay the same and it'll be sure to work with your existing phone. This said, outside of the exceptions that prove the rule, AT&T and Verizon coverage is usually a safe bet for nearly everyone if you must switch. With this in mind, I typically recommend people to only one MVNO now, RedPocket, because their prices are competitive with the rest of the industry, and they offer plans on all the major networks: AT&T (GSMA), T-Mobile (GSMT), CDMA (Verizon), and if you're feeling especially masochistic, the remains of Sprint that're still active (CDMAS). $10 gets you 1000 minutes, 1000 SMS messages, and 1GB of data, and that's not the only plan they have.
Feature Thoughts: People will boast about iPhone camera quality, but buying an iPhone means buying into an entire ecosystem of overpriced everything to make it work even remotely sanely, because the only way you're going to be able to connect and sync an iPhone with Windows outside of services like Box and Dropbox is through iTunes, which has been a dumpster fire for as long as it's been available on Windows, and Apple has no vested interest in improving the app, because a terrible experience between their phone and your operating system is an incentive to change platforms to a computer that works with the phone you just dropped a grand on. Also, iPhones don't come with SD card slots, which means no way to upgrade storage. But their OS gets officially supported longer than most Android devices, which doesn't mean much if you're buying the cheap end of used. Most Android phones, for all the problems I have with Google as a company and the platform in general, have SD card slots... and headphone jacks... and USB ports... and sometimes user replaceable batteries... and can have their screen replaced without bricking the phone... and... and...
General Recommendations: My two go-to Android phone manufacturers that I typically recommend is either Motorola or HMD/Nokia these days. Most Moto handsets have user unlockable bootloaders for developers, which is code for being able to load LineageOS onto the phone past the end of life support date, but requires some technical skill and willingness to defeat the most critical security point on the device to protect against malware. Some of their lower-end handsets even have user replaceable batteries. HMD/Nokia has some excellent phones with some amazing hardware camera setups, and are early adopters of Android Treble and upgrading handsets in general, but their bootloaders are locked down pretty tight and you can't replace batteries yourself. Both use pretty vanilla builds of Android, as well. This said, Google Pixel phones usually have good cameras, even if the rest of their build quality and lifespan leaves something to be desired for the money spent, and you're basically guaranteed 100% stock Android.
Camera Recommendations: Most smartphone cameras are punched up by image post-processing through software to cover crappy optics and sensors these days. You need good optics and sensors if you want good photos, that's not cheap when you're talking about the requirements to do so in such a small and thin package. Although there are pro-grade capable cameras packaged with smartphones, even the best smartphones still cannot do as great a job for the money as a dedicated DSLR.
Which brings us to
ACTUAL RECOMMENDATIONS:
If you're brain damaged and willing to play chicken and either stay on or go to AT&T's network to get the best for the smallest amount possible in the form factor with no other considerations, the
Lumia 1020 is what I'd recommend at around $80-100. Keep in mind this recommendation is made by a brain damaged idiot that's still using a Lumia 640 on AT&T.
For something very current and new with an excellent camera that's guaranteed Android updates for a good long while, the
Nokia 8.3 5G (also known as the
Nokia 8 V 5G UW for Verizon) is an excellent choice at $450-650.
For something in between, most flagship phones from any manufacturer have decent cameras. Samsung Galaxy S series, HMD/Nokia 8 series, Google Pixel, Moto X series, etc., are all pretty decent choices, but I wouldn't get anything older than about two years, mostly out of concern for security updates, but also trying to guarantee VoLTE support for the network you're on. This said, if you keep data off and don't use it for anything but Dropbox/Box syncing on home WiFi, that does change a couple concerns.
Hope this helps!
