To get trade velocity started in a new exchange, provide members with credit lines, much like a bank issues credit cards to businesses with credit lines.
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Members can spend the credit lines, but they have to pay back the credit line with sales. And, you can charge trade interest on negative trade balances.
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Note: Credit lines issued are not backed with cash, but you should get a credit card or bank account info on each member to charge their fees to their credit card or by EFT/ACH to their checking account. 

In your trading rules, it's a good practice to indicate that if they don’t pay back the credit line with sales on barter, then the amount of their negative balance (credit line used) will be due and payable in cash.