If you had a Buy It now, the Buy It Now amount, the Buy It Now option disappears as soon as your auction receives a bid. Do you already have a bid on your auction? At that point, you can't sell it as a Buy It Now any longer.
Exactly, once you get the first bid the buy it now disappears and bidding begins.
I like buy it now but I rarely add buy it nows after requested if I haven't put one on in the beginning. And never if I'm not 100% sure of the current resale value when somebody asks because, most of the time, it is a collector or another seller trying to buy it too cheap.
Way too often I see new sellers taken advantage of by some of the same ebay members who offer dirt cheap prices. A week or two later, I'll see that same item relisted with a more detailed description and top end pricing by the new owner.
I can't tell you how many times I've been told somebody was going out of town and couldn't bid or needed it for a birthday, ect, so would I sell it right this minute.
If I do, it's always after VERY careful research into recent prices of similar completed items. I check the bidder's feedback too, to see if they buy a lot of the type of item they're asking about and whether they sell on that id too.
To search by completed items type the name into search>then click "completed items" in box down left side of screen>then, to make it easier, sort by highest price first and it'll show you what others have ended at within the last 30 days or so.
The fact that you got a request tells me you probably have something good there. It might be fun for you to watch the bidding if you don't need to get a set price for it...