I do not know how the answer to these questions will make the development of your project faster because they seem random, but the problem of traders with accepting cryptocurrencies is related to price fluctuation and the lack of clarity of regulatory restrictions (and thus taxes.)
In short, the problem is not related to what is a cryptocurrency used, but rather crypto as a whole.

The best thing you can offer is for your site to act as a middleman, accept payment from the buyer in cryptocurrencies and pay the store by traditional methods.