The question is "will the investment in implementing cryptocurrency payments be covered by an increase in the overall revenue?". If the answer is NO, why would any e-shopping site accept crypto payments? Even if an online shopping site accept crypto, will there be enough people willing to spend BTC on online purchases? I think that the answer is NO. Also, assuming that crypto payments are non-refundable, while fiat payments are refundable, do you really believe that the people would prefer a non-refundable payment method over a refundable payment method for online purchases? I think that the answer is obvious.
I would guess it could. Think about it, is there any cost involved? No, you just implement it once, and then you do nothing about it, there is no concurrent cost to it monthly or whatever. So, what does it mean? Even if you get one sale per month, that is still profit from it, hence it is a profitable idea to add. When there is no money loss at all, and there is a chance of making any money, then that means it's a profitable idea.
All ecommerce websites will consider that, as an option. Obviously this depends on the size as well, like for example if you are just selling few hundred dollars a month, then I doubt it would make a huge change, but if ebay added it, then yeah I am sure that it will have some sort of benefit to them considering the size.