How does one so the glitch in the ship shack where you can buy something you can't afford?

You hover over the thing you want to buy and try to buy it. It'll give you the message that you can't afford it. Quickly then scroll over to something you can buy and buy it. If done right, you buy the thing you can't afford rather than the one you can

For the record, ship shack glitch pretty much doesn’t work on PS3 due to the faster loads.