buy it now
Once a bid has been placed on the listing, the Buy It Now feature disappears, then it's just a bog standard auction.
It doesn't matter what the BIN is as it will disappear when the first bid is placed.
So, if somebody bids £900 on your conservatory, the £1800 BIN will disappear and the bidding will only increase above £900 if somebody else bids more.
If it doesn't reach a figure you are happy with then you CAN'T refuse to sell without getting a black mark on your account..
Might I also recommend that you read this thread as you COULD be in danger of losing both conservatory AND money if you acept paypal or allow a buyer to send their own courier.