A seller cannot cancel a bid AFTER the item ends. But a seller can cancel any bidder's bid(s--one cancellation removes all bids by that bidder on that auction) at any time before the auction ends, and may do so for just about any reason, the verboten reasons having to do with schemes to evade eBay fees, such as:
It is specifically against the rules for a seller to cancel all bids and end the auction early without a winner IF it is because s/he deliberately set the Starting Price lower than s/he was willing to sell for, and it hasn't reached that high. This is considered fee avoidance because eBay charges a substantial fee for sellers to have a Reserve Price so they aren't promising to sell as low as their Starting Price.
But there are legitimate reasons why a seller would have to do so, and a few years ago eBay got tired of having to divine the subjective intent of a seller who took that action, so it adopted some procedures that make it less attractive to do so with that intention: it can only be done before the countdown reaches 12 hours AND it charges a fee (the FVF% for that seller in that category times the highest cancelled bid) when s/he does so (with the first such fee per calendar year waived to allow for the very occasional legitimate reason for doing so).
