Goodwill products are for sale to everyone, with no stipulations attached. Once you buy it, it is then yours to do with as you please. Including re-sale it.

At least this applies for now. No guarantees once America changes it's name to Amerika.

They do not ask for income verification at the front door of Goodwill stores. And yes, the majority of things they sell are clothing articles. So, to make money selling those, I'd imagine you would have to have a good eye for what kinds of clothes are in demand on E-Bay, and then filter through a lot of junk to find the right products. And that sounds like a useful scavenger skill, worthy of compensation.