Henry always checks http://www.abebooks.com/ for possible value too before selling things on eBay. He's had a few nice surprises!

On a totally selfish note (and, I assure you, not related to Henry's eBay), you don't happen to have a list do you of the non-Amazon-able ones? Just the authors maybe? I read some old historical romances and I'm sure I can't be the only one on your Flist (or maybe I am!).