First of all, a "crypto Ebay" would not be feeless. Handling with Bitcoin costs always fees.
Secondly, crypto Ebay's already exist in the Dark Web. There you can find many marketplaces and Ebay clones where you can buy all things and stuff (legal and illegal) with Bitcoin and cryptos like Monero.
I agree. With bitcoin, I think the fee might be lessen only a little bit if ebay will accept it as a method of payment. Also, adding that feature on their store needs a proper survey, because they will spend money on it if they want to put it on their store. If the survey turns out that there are less people who are willing to pay in Bitcoin, then there's no point on adding it on their feature, simple as that.