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