You can best guarantee trade credits by having a $0 deficit system. That means that the total positive balance for all accounts equals the total negative balance for all accounts.
You accomplish this by your exchange not spending any trade that you haven't first earned (from fees or sales).
Also, be sure to transfer a certain amount or percentage of collected trade to a reserve account each month to cover any default of trade payments by members.
If a member having a negative trade balance goes out of business, you will then be able to use Transfer Trade from the Reserve account to the member's account, so their negative trade balance becomes $0.