Rather less than they should be, sometimes. There's been a long thread on a Pistonheads forum alleging that a well-known ebay seller was taking advantage of RM's less than 100% checking by sending more items, and of greater weight, than they declared to RM. It is even suggested that the seller had "influenced" RM staff to turn a blind eye.
This may all be malicious fabrication, of course, but the seller has stopped trading and is said to be under investigation by RM, HMRC, and Uncle Tom Cobbleigh.
Others have suggested that, for a new business, it's a sound investment of a few hundred quid to sell items at a loss to get a rapid high ebay feedback rating.