With at least abebooks, and probably the others, you can register what you're interested in and get emails when items appear.
For instance, I collect books by Howard Pease, who wrote "Young Adult" seafaring stories about tramp merchant marine steamers from 1927 - c. 1962. I only want 1st editions with dust jackets -- abebooks notifies me when matches are listed. The matches are not 100% accurate, but pretty darn good. You really can't beat this!
The only downside to on-line bookselling is that since almost all serious booksellers participate, it kind of takes the fun away from going into stores on out-of-town trips, not knowing if you'll find a gem you've been searching for, because if it was there, it'd be on-line already. Kind of takes the "romance" out of bookstore prowling.
I find a lot of the books I want that are out of print. 
Linear Mode
