Lots of reasons why your items may not be selling on eBay or the other place. I would say that the most likely would be your description or title if you are using the same on both. To sell on these platforms "the title is vital". If it were on the other big website that offers Prime, it would be to do with your brand. Even if your title is rubbish, price is high and your description is naff, you can still sell if you have a million trillion visitors and a conversion rate of one million trillionth. Improve your title, graphics, keywords and description and you will improve your conversion. Advertise with all the costs that entails and you will only increase the number of visitors, not the conversion rate.