I would never sell on Amazon and eBay as a method of trading. What I would do however is utilise these as an advertising method.
Have your own ecommerce store set up and sell a selection of products via Amazon eBay etc. Ensure that it is obvious, but without breaching any T&Cs, that you have your own store. Image watermarks work well
as does your email address in any contact details or listing!
The additional cost caused by using these sites should be seen as part of your advertising budget. In fact it will probable be the best spent money in your advertising budegt.