On the secondary market page for each loan, you will see a “Create Bid” link. This allows you to specify how much of a loan you wish to buy, what price you are willing to pay and when you want the bid to expire. In order to place a bid, you will need to ensure that your account contains enough money to cover both the capital cost of a loan and one period of accrued interest. Any money that is not used in the bid is returned to your account (eg. If you bought a loan on the 15th day of a 30 day accrual period, you would pay for the 30 days upfront when you placed your bid, but receive the remaining 15 days back when the trade occurs).
Once your bid is placed, the money is removed from Available Funds until a trade occurs or it expires.