Understanding your balances.
Under each currency type in the sidebar, you'll see a large number indicating the total amount of USD, Bitcoin, or Siacoin that you have deposited in your account. Importantly, you'll also see 'Avl:' indicating the available balance of that currency.
When you bid, your bid amount will be deducted from the Avl, or available balance. The total deposited amount - the larger of the two numbers - will not change unless you deposit more funds.