That's one way to do it, but it's extremely challenging. The sales tax can depend on where the buyer is located, where the buyer is from, where the seller is located, and where the seller is from. There are ~1000 jurisdictions for each of those four choices, so that makes for ~1000^4 different cases to handle. You'd also have to register as a merchant in each juridiction, and know the rules for when you have to start remitting sales tax to that jurisdiction. There are whole businesses built around managing that complexity, e.g. Avalara, Vertex and TaxJar (now part of Stripe). Maybe just offload that burden to one of them and focus on building your core innovation.