The market value is whatever someone's willing to pay for the player. Bidders can back down whenever they feel the asking price is beyond their evaluation of the player. In that sense, the player's owner is just another bidder. I have had one or two cases where the owner kept bidding at the last minute to push up the price. After the price got out of the range I found fair for the player, I left the auction and searched for an alternative.
I agree that it's not the most ethical thing to do, but the rules are there and this is not cheating. Bidders should be disgusted by this and stop bidding, thus resulting in the owner losing money at the time (taxes) and in the future (would get smaller % of future transfer). But I think there are much more outrageous cheating schemes out there. I have been reporting a few cases lately of teams creating a bunch of new teams to buy their crappy players for 100x what they're worth.
Just an example, to sum it up. Suppose you have a player you want to sell and you think he's worth 800k. You place him in the market for 1k to get as many suitors as possible. If, let's say one hour before the end of the auction, the bid is at 300k only, I don't see any wrongdoing with the owner placing a 600k bid. If the suitors still think it's a fair value, they'll bid again. If they don't, they won't and the owner won't lose money by selling a player for a smaller sum than the one he finds fair.