Once an auction has a bid, you're not able to edit it to add a buy it now price (if you had a BIN price before someone placed a bid, the option to Buy It Now disappears and it runs as a normal auction), so if you decided to accept the offer, your only option is to cancel the current bid and end the item, then relist it at the BIN price for the other buyer. The only other thing you can do with a current auction that has bids is end it early and sell it at the current price to the current bidder.

The choice is yours, I'm not sure what those bikes typically sell for, but just keep in mind that sometimes people will ask for a BIN price because they think it's going to go higher and they want to secure the item, and the current bidder may not be too happy about it (even though you aren't technically doing anything against the rules).