Steam

Oh it's just a browser add-on. The "steam inventory helper". Still needs a bit manual labour though, but helps. One just needs to be enough of a cheapskate to that for some measly cent 😁

Not auto sell, but if you go to the badges screen in steam you can sell/buy all the cards for a badge in one go.

They have some ridiculously strict rate limiting though, so you do a few, then come back and hour later to carry on etc.