Dumb question... but is it legally okay to resell other's works?

Hi:

Joke if you want. If I posted an ad on the forums all over the web saying I'd bought a copy of Marlon Sander's course (or anyone else's for that matter) and was selling it at a profit - would he be okay with that?

Let's say I bought the package that costs over a grand and was selling it for $197. Is that okay?

People at garage sales, etc, are selling their "original" copy (such as with any printed material)... but if the material is digital?

What's to prevent me from making a copy of a digital product and keeping mine?

Then I'm reproducing the work for profit.

There is a huge difference between selling off an old paper book - and selling someone's digital product.

Joke if you want to.... scoff if you want to... but I have a legitimate question. Jokes don't make it any less so.