So after the seven years war some British troops who were in charge of surveying and inventorying abandoned french forts, found stockpiles of American made supplies. You can expect a very broke England who just fought a very long war wouldn’t take to kind to know that American’s were selling to both sides.
Also before the stamp act the other taxes that England imposed on America really weren't all that bad like the sugar tax actually went from 0.06 cents to 0.03 cents. The only difference was England actually started enforcing the tax hardcore instead of allowing the American courts who were pretty leadeant about the whole thing.
The stamp act became an issue because newspapers printers and lawyers use a lot of paper and there the last people you want to piss off lol.