You're right. GS isn't the problem. People in general are. As bad as GS can be the other options are much worse. Good luck dealing with the nasty and entitled jackasses on Facebook Marketplace or Craigslist. The customer service in smaller shops or pawn stores in my experience is worse than GS and they give lower trade in values. By the time you actually get a decent buyer (if you do) you've spent more hours dealing with idiots than it would take to actually work and buy a game system brand new.
Oh, and if you sell it yourself on Amazon or eBay and someone scams you? Tough shit. Literally no one will care and you won't be able to do anything about it.
For me it used to be:
Amazon > eBay > GameStop
I've bought, sold, and traded items on Amazon, and although it used to be my favorite avenue, the quality of Amazon's customer service has gone to crap in the past ~5 years. That and they limited individual sellers accounts substantially, to the point you can barely list anything without special permissions.
eBay is only okay if you don't have to deal with PayPal customer service. However, once a buyer files a PayPal charge back you're pretty much toast. You could have hundreds/thousands of sales and 100% positive feedback on eBay, and the buyer has zero/negative feedback, and PayPal couldn't care less and will still side with the buyer. I've managed to "win" a couple disputes with buyers, but only after hours of being on the phone with PayPal customer service, arguing with them, and presenting them with objective evidence the buyer is a scammer.
And then with craigslist and Facebook, people are entitled assholes like you said. If they see a used console for near or over the retail price 2-3 years ago, they will huff and puff and tell you that the console costs $300 new (as was the case with the PS4 Pro 2-3 years ago). They don't acknowledge that you can't get the console new for that price anymore, nor that no retail outlets are even selling it anymore. No that is too logical. Logic has no place on Craigslist, Facebook marketplace, etc.
So really that leaves only a couple other options. A big one being GameStop. And that is likely one of the reasons why GameStop is still in business. The convenience. I hate GameStop and their crappy "new" game condition (i.e. trying to pass off opened games as new), but every once in a blue moon they actually offer good trade in values. I would prefer not to shop at GameStop, namely because I want games that are actually sealed if I buy new, but if the associate hands me a copy of a game that is opened I politely ask that they give me a shrink wrapped copy. Isn't really ever an issue, unless it's a hard to come by game that may not be available everywhere. But the consoles I have bought from GameStop gave always been brand spanking new, and I've never had any issues with them.
Tldr;
GameStop sucks, but so does everybody else. They haut all suck for different reasons.