WikiGamesGuide posted the above video detailing a process that takes advantage of a glitch in how GTA Online tracks which vehicles you own and which you have already sold. By exiting into and out of a garage and selling a stolen motorcycle at just the right moment, you'll be able to repeatedly sell that bike over and over again, accruing as much cash as your patience for this convoluted scheme will allow.
As VentureBeat points out, both Microsoft and Rockstar Games are aware of this glitch, and one member of the GTAForums claims that two of his friends have been banned for taking advantage of this exploit. According to that same poster, Rockstar is hoping to ban GTA Online accounts found to have been abusing this cheat "as a statement against these antics." We've reached out to Rockstar for more.