The selection you find varies greatly with each store (I have visited stores in at least 7 locations in NYS now). As I said I have the best luck where there are college campuses close by. Our favorite stores for selection are in the Buffalo, NY area because there are so many colleges in the vicinity (we drive through there taking oldest DD to college). We haven't had as much luck with the stores in the Albany, NY area or in Dewitt.
What they buy and how much they give for it depends on a lot of variables. They may turn away some things because they already have too much in stock of a certain type of item. I know some PC's will not take designer handbags at all because of the risk of counterfeits, some do not care (you have to remember these are franchises). They also will not take Plus sized items at most stores, some do not sell/buy kids sizes if there is a "Once Upon A Child" (affiliated chain) within a certain range. As for the Ralph Lauren dress pants it is possible they thought the style was too conservative for the store clientele (just guessing). And as I said, they have certain things that they look for on clothing tags that tell them how "old" something is, they do not accept things more than 2-3 seasons old. A cashier once showed me how they tell on A&F items as an example.
Not to say I haven't had stuff turned away and then saw stuff on the rack that made me say "huh?" but as far as wear etc that is in the opinion of the person checking in the garments. The pricing etc is all done by their computers.