The system can be a first-come-first served thing, where a person puts how many points he wishes to buy/sell and then is entered in a list. Let's say nobody at one given time wants to buy points, but there are many people trying to sell points at that same time. And then, somebody tries to buy some points - the person who tried to sell their points will get their credits when their points have been sold, and if their points are unsold then they can withdraw their points from the stack.
That is, there would be a "waiting list", where whoever has had their points in the list for the longest, or at the bottom of the stack if you'd rather put it that way, will get their points sold immediately next, and if you are the last person who submitted points for trade yours will start at the end of the line.
As for exchange rates there can be one locked trade currency, such as it will always be 3 points for 1 credit. Alternatively, the value of points vs credits can fluctuate based on the market, like supply and demand - if there are billions of credits stored up in the waiting list and nobody submits points to sell, then the value of points will increase, and you will get less points for your money. This will make players need to be more vigilant of the market, and it would bring negatives as well as positives compared to a locked exhange rate. To soften up a fluctuating value, players can alter their offers without having their places in the line reset.