A good way of determining what you'll be receiving from GameStop is by either asking them in person or calling the store up for each item in your list. Given its length though, I'm not sure they'd be able to go over every detail, at least not on the phone.
But, if I had to guess what you'd be getting, it might come out to around... roughly in the $400 area. This could depend on if you're a card member with the store and also whether they have a special trade-in deal or not. While you could obviously sell your stuff on eBay or somewhere similar, that of course would depend on whether anyone is actually looking to buy any of the items in question. You could mark the content to sell for whatever you want... but will someone really want to spend $900 on everything on your list? You could try, but I would think you'd have a better chance selling a PlayStation 4 on eBay for that kind of money now than you would your PlayStation 3.
Game Stop is still doing the promotion for giving 50% more trade in credit, if you happened to have picked up a coupon last month (I hope you have one). Else, they are giving 30% more credit (thats the current promotion for trade ins).
I recently traded in 14 games and got over $400. That paid off my PS4.
Congratulations, you just bought a PS4 for about $840.