Partner worked in two s/h bookshops OZ/UK. The painful part is when they are amateur, and don't deal fairly with you over your own stocking and shelving policies. Leaving a trail of badly re-shelved books behind is trashing your farm. The good part is that you wise up and realize you have to do more input work on your stock to price it, if you don't want to become part of the wholesale chain. Or, learn to live with it and accept a different profit model.
Now me, as a customer? I walked out of one shop here in brisbane when I found a treasure (architects drawings from pugins practice, neoclassical sketches of British buildings) and they refused to honour the written price which was (I acknowledge) undervaluing the book, but none the less reflected what somebody had thought the book was worth, righ up until I decided to buy it. Yes, this is strictly illegal. its also impossible to police in practice. I should have reported them, I just decided never to shop there again.
Ex seller of huge numbers of books (we're kindle now. books are dirty and feed roaches in the tropics) -I salt the valuable ones in amongst the trash, because I want you, the book buyer, to buy the box not the ones you want of high worth. There have to be a lot of high worth books, to make it worth my time to sort them out, and I'm left with the remove-the-trash problem I was motivated with in the first place.
This is the deal: you buy all my books and I accept I get less than high water mark price for the jewels, but I do get my shelves emptied.