I did the exact same thing once by accident.
I offered $2,000 for a domain name privately and the seller accepted. I didn't even know it was on GoDaddy auctions at the time, but apparently the bidding was at $1,000 with a BIN of $18,000 at the time I made my private offer.
A week after the purchase one of the bidders on the auction came and offered me $18,000 for it.