Well, it's like that to discourage people from buying and selling to and from the market. It also allows players to sell items that they obtain for less than what the server market sells items for to players. Meanwhile the sell price is lower so players can sell items to other players for prices higher than the server market price. It is setup to discourage players from using the server market, and trading with each other instead.
All I am suggesting is that a single item be given an identical buy and sell price so it can be used as an item currency for player's vanilla shops.