I am an Indian ebay seller so I don't know much about .com or .co.uk, etc; but I think that if ebay suspects something wrong is being done, then they will investigate. One of my friends began selling things in almost dozens (a day!) first, then his sales zoomed up a lot. He is still selling a lot, almost 1 year later now, and I think ebay is quite happy with his performance. Of course, they must have done their secret investigations many times. So may be, AFAIK there wouldn't be too much harm if he sells too much. Take care that the feedbacks are positive though, and take care there aren't too many returns, or complaints about the products. And your friend shouldn't t sell anything that could get him in trouble. I don't know about seller's restrictions. Ebay may place a limit or restrict your friend's seller account, for whatever reasons, and may ask him to provide certain related information to lift it.
Google this (add your own variations) and read some of the results:
ebay seller's restrictions too many sales
