Interesting question..IMO it depends on how much ebay are making off you..I was looking at a seller yesterday who has sold 335000 items over the last year. In that time he has racked up 3536 neutrals and 4045 negs.  If they are making enough money off you it would appear your allowed to rack up as many as you like.There were306 negs just in the last month on 3500 sales lol. It's amazing what some folks can get away with just as long as EBay are making good money off them.