I have no trust issues with GB themselves. Some sellers are better than others, that's what the feedback system is for. I do think they are too lenient in their grading of sellers however. There are lots of sellers there with crappy feedback that still have A's and A+'s. But buyers should "beware" there like anywhere and read a sellers feedback before bidding.
In these recent cases of listing things that are already sold, its baloney and the sellers really need to stop being so lazy and get with the program.
But beyond that I don't really know what else can be done, Caveat Emptor.