After a few more bids from a few of those third years Frank had been thinking of, Rita finally got the hint that she needed to close the bidding and just sell him off to Alice, who countered every bid.
"The bid stands at 3 Galleons," she announced. "Do I have any other bids?"
Of course the third years whimpered because they couldn't bid anymore and they obviously weren't about to share their money just to get him.
"If there are no more takers, then," she grinned. "Frank, you are sold to Alice for 3 Galleons."