you will need a print out to provide for the COE. You will also need a print out to bring with you. When I entered thailand they checked every document no less then 10x at the airports.
I agree with the other poster said here about the market.. If you show up there with a seller.. You will draw attention and some will check you... And even though you might not technically be doing anything wrong, it likely won't end well.
There is no way you could get a work permit for that.. So no visa.
As long as it's not a formal business operation (like backyard garden) and she is not paying you for the help it's okay... But it might look suspicious to immagration and that could land you in hot water. As long as you can prove that you support yourself and not working here you probably be okay. You need a visa another way of course.