I'm a big ebay fan, but I was amazed at their total indifference when I provided chapter and verse on a seller who was clearly selling a succession of stolen bikes. "Are you a policeman or acting in a similar official capacity?" they asked. I said no, I was just a person. They said they didn't act on any reports from anyone not 'acting in an official capacity', and that was that. And yes, the guy carried on selling stolen bikes (roughly one a day on average). (And ebay, of course, carried on getting their 10%.)