If they bought all 50 in one transaction, they can only give one feedback. If they bought them individually, you should be making a little bit extra on individual shipping? At any rate, you seem to have covered yourself with ebay so you should not be at a disadvantage if they try to get back at you for offering a similar product. What is the worst they can do, say their widget is better than your widget? They have no patent or license to fall back on?