I'm gamesharing a few games for a gamshare in return. Destiny Call of Duty: Advanced warfare Call of Duty: Black ops (+ Dlc) Minecraft Assassin's Creed II Any game is good, but if you offer Black ops 2 you can have 2 of the above games.
Gamesharing is where two users share their login information with each other and log into their game console and download the purchased games. Sounds risky of course, but if you take the correct precautions and trade with trusted users, it should go smoothly. But I think there is a limit on how many different consoles it could be downloaded on before you'd HAVE to purchase the game/DLC again. I use to do this all the time with my friends to get LittleBigPlanet DLC on my Playstation 3.
Yes, only items purchased on the Playstation Store will work. Well, anything downloaded from the Store that is... I'm pretty sure if you enter a code found in a game case into the Store, that would be eligible for Gamesharing. Sony kinda frowns on Gamesharing.