PB~I sold some old playbooks a while back and they sold fairly quickly compared to other items like tableware, etc. that has been sitting around for a while. It seems when something is rare enough or not so common in the book world, there will often be the right buyer out there somewhere looking for it who will buy it from you if they haven't had any luck finding it anywhere else, or if your price is right for them.
Do you have a blog that promotes your subject(s)? This may also help build you a reputation for offering books on certain subjects.
Also, I've been off in Pinterest land lately, and I see how using it may also bring some attention to your subject.