I use the adds in Shutterbug as a guide for what to pay on Ebay. Places such as Midwest Photo Exchange, Badger Graphics, Lens and Repro always have great reviews for their knowledge, service and return policies. So unless the price is somewhat less then these vendors I would buy from them.
I have never had a problem with purchasing on Ebay. Some guidelines to follow:
Always buy from someone who has a return policy or a trial time for the item. You will have to pay shipping to return, but that is a normal cost .
Always buy from someone who has had a good number of transactions and has positive feedback. I am leary of buying from first time sellers and never buy from anyone with more than one negative feedback in the last month.
Don't buy from overseas unless you understand the possible additional costs that may be involved with taxes, import duties etc.
Always email the seller with any questions no matter how trivial.