Too many people confuse "bid" and "Best Offer." Those are two entirely different things that the seller handles differently. People who come here and use the wrong term (what does the message or display ACTUALLY say you got?) often get the wrong answer (actually it's the right answer for the question asked but not what s/he needed to know).
A bid is part of an auction. Sellers don't "accept" them (eBay accepts them if they are valid bids: meet the minimum bid requirement as of the time they are received, received before the end of the last second of the auction, buyer is not prohibited from bidding on that auction because of a restriction by the seller or eBay). A seller can cancel any bid up until the auction ends (or eBay might if the buyer is suspended before the auction ends or the bidder might retract the bid for a very short very specific list of valid reasons) but otherwise the bid remains active until the auction ends, at which time a winner and price is set from the bids that were accepted by eBay and not cancelled/retracted before the end. The seller is obligated to sell for that price (if the buyer pays, provides valid address that seller agreed to ship to etc.) and the buyer is obligated to buy. While a seller can end the auction early at any time to sell the item to the current leading bidder at the "current bid" amount (the same as if it ended at the scheduled time with no further activity) that is generally not a good idea since many knowledgeable bidders don't bid until near the end (the option is best used for time-sensitive items and/or those with a natural or artificial price cap--event tickets being the best example of both).
A Best Offer is simply an offer that the seller can take or leave. S/he can accept it, reject it, make a counteroffer (which acts as a rejection of the offer), or do nothing and the offer expires (at the duration the buyer chose when making it up to 48 hours, or earlier if there is a "bid" placed--Best Offer is no longer available after the first bid on the auction, or when the listing ends for whatever reason). The seller is under no obligation to accept any offer or respond in any way to a particular offer or any offer at all. Note also that if the offer is expired (e.g. the auction got a bid) then it cannot be accepted (either there is no link to respond or it doesn't' work).
