Obay

Gangster's Ebay. There are tons of things that can be bought and sold here. You can find Packs of Bullets, from 1000-3000 Bullets each, Cars, Casinos, Witness Statements, Donating Codes (people spend RL money to buy them and sell them there to make Omerta money) and even Families can be sold there (they usually aren't though).

Warning: You can't bid on something in the last 3 minutes, so the last possible bid should be when the item has 3M0s left on it.