I wouldnt have told them either.
There is a reason for a reserve, buy it now, and regular bids.
I would have said if you would like to purchase the product right now I have a buy it now price.
he is trying to see how low he can go on you. He's fishing that is all. Some people may disclose it. Thats up to each individual. I wouldnt. Disclosing that price means thats the minimum you will take for it so there is no reason for a buy it now price. Kinda like asking someone at a store what their wholesale price is compaired to what its priced.
IMO you did it correct.