Anyone know of a site that will give me a rough idea of what some books are worth ?

Ive got a decent collection of Dr Who novels and Im interested to see what theyre worth these days, some are very rare (I know that from a collectors book/catalogue a few years back, but cant find it now,and also a friends brother who is a Dr Who nut drools over some of them) and collectable.

Looking on Trademe dosent really help, and I doubt Ebay will be any beter but will be having a nosey.......

TIA