Here is why I think it's impossible to make a Crypto only Ebay. You need a crytical mass of users to buy and sell or you will not have a market and when you start it and 99% of the population can't use it since they don't know or use crypto how can you get a critical mass of users?

I think what can work is to make a Fiat + Crypto ebay clone and lower fees and that would work with good marketing.