MCS is a great options as mentioned already.
Instead of just selling them the entire lot at a fraction the value, you could consign them through MCS and only give up 10% (or I think it's 8% if the book's value is $300 or more).
You set the prices, and when the books sell, MCS sends you a check in a couple weeks.
Once MCS has all the books, you don't have to pack and ship each one individually by yourself. This way, you might be slowly parting the books out, but still recouping most of the equity in the books.