Don't. By that I mean, do not get involved in the tax burden yourself. Let the sellers, which are businesses, manage their own taxes. This is what I am doing with my own project. Amazon and few others have taken up on themselves to make life of their sellers easier by handling taxes for them, especially for EU, but i see no reason for it whatsoever.

Businesses should be businesses and handle their own taxes and legal obligations by themselves.

Additionally, if you get involved in between the seller and the buyer, YOU are now liable when it comes to taxes, as you become de-facto the seller, you have to handle returns/refunds(including the tax aspect and have to handle already paid taxes in such cases as well), audits and other legal aspects. There is simply no good reason why you should involve yourself in this whatsoever. Just facilitate the business transaction, but not the financial transaction.

Related "Ask HN: How do you handle VAT / Sales Tax accounting as B2C SaaS?" https://news.ycombinator.com/item?id=43298663