Well consider this message which is displayed before you sell your player:
If you feel you have received an unreasonable sale price for this player (or just want to be on the safe side), it is your responsibility to make a formal report as soon as possible.
The player in question was sold for an unreasonably low price, which unfairly hurts the seller, while rewarding the buyer. Anyways, the reason the rules only explicitly mention overbidding is because underbids are prevented by the market: if a player's price is too low, other users will raise the price. The reason it didn't happen in this case is because of server issues.