It looks like a reasonable iPhone 7 would go for $320-400 on eBay. The 8 bumps that to about $600! Oof. Don't let a raise with a new employer inflate your lifestyle :)
My reverse chronological order of used phones is this:
Pixel 2 XL - for spouse (Retail $850, eBay $470)
Nexus 6P (Retail $550, eBay $290)
Nexus 6 (Retail $650, eBay $425)
Every time, the condition has been damn close to new. (Read the description, check the seller, view the photos.)
Personally I recommend Consumer Cellular for all but the heaviest of mobile data users. They use AT&T and T-Mobile networks, so you have excellent signal damn near everywhere, and the prices are really good. The customer service is excellent, and they just bump you to a $10 higher plan if you really need more data for some reason. (You can bump yourself back down the following month.) MvR and I share a plan and pay $62/month all in.