Something I haven't seen mentioned in this thread (or maybe I just missed it)
is that Half.com is free to list on, and once one is familiar with Excel or
some other such means of producing appropriate uploads to Half.com, maintaining
an inventory on it takes only 5 minutes a day, max. Although it generates
fewer sales for me than ABE, Amazon, and even Alibris, I don't LOSE money
listing my inventory there and the time I do spend on it is more than
reimbursed by even the meagre sales it generates.

Just a thought.
Frank Jason Oreto
Eljay's Used Books
Pittsburgh's South Side