Given that you've already received bids for the item you've got no means of being able to accept an offer. If you'd listed the item with the Make Offer option included but had not received any bids then the person in question could have submitted his/her offer via the Make Offer option, and if you'd seen the offer before anybody else had placed a bid you could have accepted it at that point. However, once a buyer places a bid the Make Offer option vanishes from the ad, so unless you were to cancel the ad and relist it at the price that the person in question offered you, in which case you'd be charged 10% of the highest bid, the only way that the person who offered you a set price for the item could get it now would be to submit the highest bid and pay up after winning the auction. However, it would be worth bearing in mind that a lot of chancers do this in order to try and get the item for a cheaper price than it may have sold for if the auction had been allowed to run its course, in which case it wouldn't be in your best interests to cut the auction short if it could potentially go for more money than you've been offered.
At the end of the day it's up to you as to what you decide to do in order to deal with this matter, but personally if I had placed a bid on an item and the seller cancelled my bid and ended the listing with the specific intention of selling the item to another person then I would make a note of that seller's User ID and make a point of avoiding buying anything from that seller again in the future, as personally I think it's extremely out of order to cancel the bids of people who've shown that they are serious about wanting to buy the item by committing to placing a bid for it, just to sell to a chancer who may or may not pay up. It's sellers who do things like this that give eBay a bad name, and although some may feel that I'm being a bit harsh, I believe that if eBay catch sellers doing this then they should kick them off of the site for good, both as a buyer and as a seller, so that they can't disrupt any further auctions. If bidders lose out to another person who won the auction fair and square then that's not so bad, but if they lost the auction because the seller cut the auction short to sell to a chancer who bypassed the auction by offering the seller a set amount without even having the decency to place a bid for the item then that just makes a mockery of having a system in place whereby the item ends up being sold to the person who ends up placing the highest bid. Why even have the Auction format at all if it can be so easily bypassed by a persuasive chancer who can talk the seller into cancelling the auction to sell the item to him/her?
