It can be done, but how to do it wil depend on the negotiated price.
You can end the item and choose to sell to the current highest bidder, which will mean they win the item at the price the item currently is (so $150 for that stereo system). If a different price has been negotiated, you will need to cancel their bid - I'm not 100% sure if you can just add a buy it now price to the auction once their bid has been cancelled (you can't add it while there is a bid), but if you can't add one after cancelling their bid, you will have to end the listing, then relist as a Buy It Now at the agreed price.
