At the Lego store where I work (I won't say which one) we only rarely put out grab bags. The parts mostly come from the PaB wall. When there aren't enough parts in a bin on the wall to make it look nicely filled and we don't have any more of that part in the back, it goes into a big bin we have in the back. When that is full, we empty it into grab bags (as people have noted, we usually do two large cups full per bag) and put them out. Every time I've seen them put out they have all sold that day or the next. While we do put damaged sets in there, it is only if none of the employees want them. It is VERY rare that a licensed set, ESPECIALLY star wars, will make it into the bin. Furthermore, most returns, at least at my store, are not damaged and go right back on the shelf.
