The cause is asymmetric information. Only the seller knows the condition a writing instrument is really in; the bidder can neither handle nor try it and has to estimate its value from indicators: photographs, the description, the seller's feedback ratings and further points of reference. The better these indicators reduce the uncertainty, the higher the willingness to pay. Where they are absent, bidders factor in a risk discount, and the piece goes for less than it is worth.
That explains why auctions, for all their efficiency, do not produce uniform prices. The form of auction used on the large platforms corresponds to a variant of the Vickrey auction: the bidder gives the bidding agent his maximum bid, that is, his true willingness to pay, and the agent automatically raises the bid only as far as is necessary to outbid the others. Anyone who knows the mechanism therefore bids once, at the amount the piece is really worth to him.