It's only dodgy if you offer it to the second highest bidder at their maximum, instead of what they would have won it at if the non-paying bidder hadn't bidded. The potential buyer would probably be so impressed that it wasn't just shill bidding to get their maximum bid, they are likely to jump all over your item.
At least I would, if I was ever offered what my winning bid amount would have been instead of my max bid amount.