All offers become void once a bid is placed. The laws governing the conducting of auction in some states could be violated if the seller accepted an offer once a bid has been placed. While eBay, being only a venue, may not be in legal jeopardy, the seller may very well be.
When items are being sold at auction, bidders have the right to expect that the auction is being conducted in good faith. That is why many states have strict laws governing them, including licensing requirements, for auctioneers and auction businesses. Frankly, eBay's decision to include the PO option in auction listings was, in my opinion, ill advised.
Since a bid was placed, any previously made but unaccepted offers are void and the BO option is deactivated. That means that anything that you negotiated with the person making the offer is off the table.
The only way to accept an offer on it now would require that the auction be ended without a winner and the item be re-listed in the fixed price format. Doing this, however, would definitely damage the seller's reputation among those that like auctions.
Needless to say, I would advise anyone holding an auctioneer or auction business license to be very careful to check their state's auction laws before entertaining any offer outside of the normal bidding process. The loss of one's license usually wouldn't be worth the risk.
"It is an intelligent man that is aware of his own ignorance."
