I've had ~300 transaction buying Superpoints on Ricardo - so far two have gone wrong. But in both cases, I have received either my money back or my points in the end.

The last case was just resolved a couple of days ago. I did get my points after paying the seller a visit at his home. But there are other buyers that I know of who have paid and are still waiting for their points from him. At least the guy has now been banned from Ricardo.