A search of completed listings can be a useful tool as a value guide, BUT realize games can be played that affect the accuracy of the results. You wish to list an item and have no idea of market value. Do an ebay search of current listings of a similiar item. To the left of current offerings is a margin in yellow. Scroll down to search options and then to completed listings. Place a check in the completed listings box. The listings of the past 3 weeks or so will display. In the price column the prices will be in red or green. Red indicates an unsold item and green indicates a sold item. Both red and green prices may have merit, but realize that anyone can ask anything for an item. An unsold item could mean on that day there were no takers or that the price does not reflect value. Red prices items may or may not be reflective of market value. Green prices reflect an actual sale and are a more accurate indicator of value BUT realize many factors and games may be involved. Factors affecting value can be location, accepted payment methods, seller reputation and unique details of the specific items. A BIG factor are the games played by sellers/buyers for the purpose of rigging auctions. A seller may offer an item with a low starting price and no reserve with no intention of allowing a fair auction. This allows for low listing fees to the seller. If the item does not reach a predetermined value the seller will have a friend place a phony winning bid. After the sale the buyer and seller will cancel the auction by mutual agreement. The seller hopes to create an artificial high market that may influence a second chance buyer or the next auction. Still, the item shows in completed sales in the green column and can be an improper guide to value.
Say you are trying to establish market value on your 1999 Toyota Camry. If a search of completed listing shows a dozen sold in the $5,000 range and one sold for $12,000, unless there was something very special about the higher car (or yours) the completed sales listing would imply your car (subject to factors) would most likely sell around $5000.
The completed listings feature of Ebay sales can be a useful guide to determining value IF your are aware of the pitfalls.