Myself and my wife (we are 40) have EXCLUSIVELY bought used phones from Ebay and Facebook marketplace for over a decade. I 100% recommend it.
On that note here is my playbook....
1.) Buy flagship phones ONLY....so Samsung Galaxy, Galaxy plus or Galaxy Ultra, Galaxy FE is ok too IF you just need a basic smart phone (not recommended for a teen), Iphone, Iphone Pro, Iphone Max, iphone SE is ok (not recommended for a teen/young person), Pixel or Pixel Pro. DO NOT buy most other phones because they are made out of cheaper components and the operating systems will not be supported as long as flagship phones because they didn't produce as many. Flagship phones are like the toyota's/hondas of the phone world. They will get the most miles and have the least issues. When an issue arises, someone 100% had that issue before you and you can easily find a solution on google.
2.)While I did say you can buy the Samsung Galaxy, Google Pixel or Iphone.....opt for the higher end models (Galaxy plus/ultra, iphone pro or max, pixel pro). WHY...because as phones age, their batteries wear out. I have tried several times to replace a battery and it is VERY easy to mess up a phone doing it. Thus, this higher end models tend to have batteries that are 20%+ larger and will not require as frequent charging when they are 3-7 years old. 20% doesnt seem like a big bump, but when the battery is 20-30% worn (holds 20-30% less), you really notice the bigger battery.
Also this top model or two often has more system RAM, larger screens, and may have a slightly better camera
Also...because I recommend buying older phones (see below) the large price difference between the base model and the 1 or 2 higher end models is not so big because the expensive phones are 70% depreciated.
*if buying iphone...do the battery research on battery life in hours...the difference is a little less drastic than the battery difference with galaxy/pixels.
3.) Pricing sweet spot....I found buy the 3rd or 4th generation back (with a strong emphasis on the 4th gen) to be the sweet spot. Example...the Galaxy S24 just came out (current generation), used Galaxy S23's are getting cheaper but I still don't want to spend $400-$600 on used entry level flagship phone.
But the Galaxy S22 PLUS (3rd generation back), I can buy "excellent condition" refurbished for under $400. The S22 Ultra is $500 (id lean to the plus, the battery ISNT THAT much bigger and the camera isn't THAT much better.
WHAT if I really wanted the trumped up best of the best camera.....go back 1 more year. Galaxy s21 Ultra (4th generation back) is $350 (new S24 Ultra $1300...so that is 74% off)! A Galaxy S21 Plus - $270 (new those are $1000, so 73% off)! The galaxy s21 base model can be picked up used on Amazon for $240ish (new they are 800, so 70% less). AGAIN...maybe the $110 extra bucks to go from the base model to the Ultra isn't worth it unless you REALLY take a lot of photos BUT ....THE EXTRA $30 bucks to go from the base to the galaxy plus is 100% WORTH IT.
If you are looking at iphones, shop for an older used Iphone Pro. About 4300 on amazon
4. Where to buy - if you dont know what you are doing and want the least risk ... go Amazon. Prices are fair and youll get the best customer service if you have an issue.
If you are good at reading fine print...you may save an extra 15-25% buying on Ebay. HOWEVER be careful as the listing can be a little more confusing and you really have to dig around. Youll see a phone for $100 less...open the listing only to find to get 100 bucks off you need to buy a heavily work or parts only model and the excellent condition was the same as Amazon. There ARE deals on ebay though, you HAVE to shop around. Dont buy ANYTHING from anyone with under 500 ratings and 98%+ positive feedback. 4/5 times I buy from ebay. I could find a galaxy s21 plus for about $225
Last there is Facebook marketplace....you CAN get a deal but you have the highest risk here. You HAVE to haggle a fair price. For marketplace I want to pay 30% less than I would pay on Amazon (only slightly less than I would pay on ebay....but you don't pay tax..so that's where the savings are). I would message 10 phones....offer them $20 under what you want to pay and 1 or 2 might bite. So I wold offer $170 (for a phone I could get on amazon for $270) and settle at $190. There is likely no return policy here or store to report. I have done it a couple times...been lucky enough to have 0 issues so far but I need to get a good deal to be worth the hassel. 30% off amazon is my threshold.
5.) condition - buy excellent or "minor" or "a condition" wear. A few very light scratches on the screen or a couple dings on the side is NO big deal. Plus you will likely put on a case / screen protector anyway. I MIGHT consider "b" condition for a great price. Hard no on c or d condition / major wear / heavy wear.
6.) Buy a phone with 5g...I think the iphone 11 are not all equipped with 5g...so in that case I would lean to the iphone 12.
7.) Cameras - honestly...NOT that much difference from a camera 4 years old and today. Also...not THAT much difference between photos on a top model and a base. top model cameras have better zoom and newer cameras have better image processing....but if you look back in a decade....you wont notice a ton of difference in the photos of an iphone 11 base vs iphone 15 pro max as your are scrolling through. If you are a photographer, influencer or have small kids and take 2000 pictures a year....splurge $100 more on the top model from 4 generations back.
8.) Pro tip - save an extra 10% by picking out the least desirable color. My last phone was hot pink...but the cover covered 99.4% of the pink and youd never know. Purple or pink phones sell for less and I rarely if ever take of fthe case.