If you want a decent idea of the value of a used marker go to the Blue book here > MCB Paintball - Paintball's Official Blue Book
Ebay is for well informed buyers. If you must go to Ebay be patient. Like any other auction house the prices reflect the ignorance of the patrons. Decide ahead of bidding what your maximum bid will be and stick to it. If you dont get the item you wanted theres another just around the corner, probably at a better price.
The buy and sells on sites dedicated to paintball have pricing that better reflects the going rates for paintball equipment. Even the Blue book is not 100% current and as far as AGD products are concerned I find the Blue book a bit high.
Good luck in your search.