Transactions will not process if the transaction amount exceeds the available balance (trade available + credit line).
Charging interest on negative balances is a good thing UNLESS you want to promote interest free credit lines, which is an enticement to join an exchange.
Interest on negative balances are set to an annual percentage, such as 12%. You can set it as a default member fee that is automatically assigned to each member when the account is added, and you can change the interest rate or remove it on a member by member basis. Then when you run monthly processing the system will automatically charge 1% trade interest on the negative balance amount.
When you assign credit lines to members, you are not giving them trade dollars you have or haven't earned. The credit line is the same as a credit line on a credit card, whereby a cardholder can spend up to their credit line. The benefit with a barter exchange, is that the credit line is paid back with sales as compared to a credit card that requires sending a payment.
If a member spends their credit line, or attempts to buy a marketplace listing for an amount greater than the trade available (trade balance + credit line), the transaction is declined. The transaction should display in pending trades for you to approve it or decline it.