Bob, what Skip says is sound advice. It realy pays to know the marked and to know a lot about the item you are looking for. Only then are you ready to find that great deal. So keeping an eye on Ebay and brush up on your m3-knowledge.
I found a lot of valuable information in Coniglio & Ladens book (the one Skip also mentioned).

Good luck & Happy hunting!

(the hunt is halve the fun...)