Do you mean a listing that is only for one person to buy? If so, just use a "code" title for the listing like SSfg63. Also, make it an auction not a buy it now. that way if another person bids, you can cancel their bid.
If you mean an auction for one person, that's no longer available. What I do is change the listing to add best offer. If the listing was originally for $30, I'll change the price to $50 and add best offer. Then tell the prospective buyer to offer X amount and I'll accept it.
