Income taxes, sure, but for a while nobody had to pay sales tax on out-of-state internet sales. I could see someone not realizing that had changed.

Theoretically they should have gotten good help from a CPA by then, but if it's a largely automated business and it suddenly skyrockets, it might be easy to just enjoy the money rolling in without taking it seriously enough.

My understanding is that it used to be the buyers' responsibility to pay taxes (so almost no one did), and then laws were passed to make it the responsibility of the seller to collect the taxes.