I am not an acountant / this is not legal advice/ yadda yadda yadda
I dont believe you do need to pay sales tax (in the US). The reason is you are not the one conducting the sale, the vendor (Apple, Google) is the one conducting the sale. The money you get would technically be royalties, not sales revenue. They are the ones selling the app, and you are getting X per copy - much like an author would from a publisher. Obviously royalties would be a factor in income taxes.
Google, Apple, or the Buyer (via a Use Tax), on the other hand, would be the ones who were responsible for paying sales taxes.
Outside the US... have no idea. Would depend on local laws.