I built something like this way back in 2008, since paypal let you set a IPN dynamically. Was basically a pastebin behind a paywall. I found it incredibly difficult to market, but I think I was doing it wrong because gumroad took off many years later

https://www.redferret.net/tiny-checkout-very-cool-little-onl... https://web.archive.org/web/20090227044608/http://www.tinych...