I think I've learned part of the alchemy of selling to Half-Price Books -- skip the paperbacks, be sure you have four or five current titles and keep the overall number at about 20 or less.
The bottom line is that if you give them stuff they already have lots of, they won't offer you nearly as much as you think they should. That doesn't always work, of course. I brought in a good load of stuff I use in my church work, and the two workers making fun of it didn't know I was in the aisle next to them. I didn't hear them say this, so I can't be sure, but I think I got a lowball offer in order to see how I would react (these two clerks were more surly than professional in a couple of other interactions I'd had with them before). I took it happily -- the key was for them to get rid of the books for me, and any money was more money than I walked in with -- and I think I may not have made their day.
Today's clerks, by contrast, were professional, courteous and cheerful. That's usually the rule at HPB; the other pair were the exception.