I went to four different Gamesstops in three days trying to figure out how to get the best deal since I was willing to trade two PS3s (a 320GB slim and a 250GB super slim).
Gamestop #1: Any PS3 model (even the fat one, which was surprising to me) is accepted as long as the hard drive is at least 120GB. I forgot to ask about trading in two PS3s towards one PS4.
Gamestop #2: Fat PS3 models are not accepted at all. One PS3 would be accepted at $175, the other at the regular rate.
Gamestop #3: Again, fat PS3s are not accepted. The employee was unsure if two PS3s could be scanned towards one PS4, but she was willing to try. I had to backup my data and deactivate the systems so I couldn't do it then.
I go back to this store the next day and a different employee is there. He will only accept one PS3 (or trade them in separately). I give him the slim. He turns it on and fiddles with the menus, but doesn't try out a game. He says I'll get $137 because the system and controller are not in very good condition. Both are in better shape than the used systems they sell, so I said forget it and took it back.
Gamestop #4: At this point I was no longer interested in selling a fat PS3 so I didn't ask about it. Over the phone, the employee is not sure about PS3s towards one PS4, but says it should work. He turns on both systems and tries out a game to make sure to both work, he even connected them using the composite cables I gave him. No issues what so ever.
It went much better that I could have hoped because I only had to pay tax on $49.99 instead of the full price. Not bad at all.