When you bid on or withdraw an item, the coins will be instantly removed and only returned if the trade fails. You'll just be left with balance you can actually use on bets or more withdrawals, rather than all your balance combined into one.
Example:
You have 100 coins, and you place a 50 coin bid on an item. When you place the bid, you now have 50 coins in your balance.
If you win the auction and receive the item, you're left with 50 coins. If the auction fails or you get out-bid, you get your 50 coins back and you're left with 100 coins.