Most bookstores will not stock anything that is nonreturnable. Returns started as a desperation sales deal during the Depression, and publishers have never been able to back away from them successfully since.
Yes, one source of bargain books has been books printed solely to be bargains. When I worked at a chain bookstore, I bought some of those titles that typically cost a dollar or two, no more. Some fascinating finds about obscure things, probably from preexisting files from the UK or the like, so in essence a cheap reprint that they didn't think would get any play any other way.
You can occasionally see big dumps of similar books in grocery stores; those tend to be paperbacks I'd give a B or C grade as to content.
The other major source was the remainder companies, which routinely returned hardcover bestsellers to us to sell at bargain prices. Those tended to be (back then) about 33-50% of the discounted price of the book when it first came out. Not the cover price.