Putting your software on the App Store or Steam or similar (notably not Google Play[1]) means you're just getting paid royalties, not selling a product to individual customers.
[1] https://play.google.com/intl/ALL_us/about/developer-distribu...
"You are the merchant of record for Products you sell through the Store." I'd bet that 95% of smaller Android app developers have not hired an accountant to review every last detail, and therefore have failed to pay every cent of tax they owe. Unless you're very serious about starting a real business, it's a total nightmare.