As mentioned, generally you should be safe dealing with GB however you just need to be more careful with sellers (or buyers if/when you're on the other side of the fence) that have very little reviews. Generally, known sellers, those with hundreds or 1000s reviews, generally ask for a US Post Office MO as those cannot be rescinded. They're as good as cash, unlike bank cashier's checks or money orders that need to clear first. It's reasonable that a seller doesn't want to release their merchandise until they know they have money in the bank. And 99.9% covet their ratings so this is your hold on them if performance doesn't happen.
All said, sometimes you get good prices on GB. Much of the time you don't. The danger is getting caught up in the bidding process. It can get exciting so keep the emotions in check. Nothing better than to watch a piece up for sale on GB and see 2-3 bidders just bidding it out of sight in pursuit of the win. It can be addicting. Good luck!