Hi, niche markets, and items people fight over particularly collectables which can be listed for say 99p or 99 cents and sell for £100 or more. So minimum insertion fees, sell first time everytime
Alternatively sell mediun weight items with low fees and low profit margin, in bulk everyday.
Trouble is unless your knowledge/interests are in tune with a product hungry market it is tricky, I reckon if I spent more time selling stuff I thought was a total waste of space but are major must haves I would make a bundle too