If a buyer uses the buy now option, then yes, you have to sell it to them for that price.
If you're not sure you want to sell if for that price you shouldn't offer buyers this option.
Buy now prices are a bit of a nonsense anyway, because as soon as any bid is placed, however low, the buy now price disappears. It's hard to see what purpose it serves, except to net another 50p in fees for eBay.
If you don't want to sell below a particular price, the best and cheapest way to do this is to start the auction at the lowest price you would accept.
