The economy in Borderlands almost doesn't exist. There is no way to exchange weapons; you have to drop them on the ground to give them to others. This really isn't a problem, however, as you can duplicate any item VERY easily so trading is almost pointless anyway. To do this a player joins a host with the item the host wants. After that, the player drops the item and then turns off their PS3 before the game can save.
On top of all of that, because the character save files are stored clientside, people can actually export their character save files to a flash drive, import them on their computer, edit their character using a program (give your character more health, levels, weapons, etc. that shouldn't ever exist in game), and then import it back on their PS3. Alot of people now have "hacked" gear and levels.
Despite all of this, the PVP arenas are extremely fun and the game is worth the buy anyway.