There used to be a publication, Book Auction Records, that we used rather like Glass is used in the motor trade. Not sure it is still published, ask at your library. But, looking up Abebooks, Alibris, and Amazon will give you a fair idea. Incidentally is there a copy of Hjortspring, Warfare and Sacrifice in Early Europe by Randsborg in the collection? I have been trying to find it.
Any online systems the traders use?