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.
I am in the same situation. I am here checking it out on the STV first...
If I want to come back I will probably do a study Thai ED visa... Which can let you stay up to 18 months I think. Then I can commit to the elite. Elite is just a little expensive out of the gate unless you are sure you going to want to stay long term.
You can also look at opening a branch office here for your company if you can run enough of the business here to meet the requirements. Then you can sponsor yourself.
You typically apply for a visa in the country you are in...but you never said what kind of visa ie the requirements... So I am assuming you are in the US and that is why a background check in the US is required.
I am US citizen... Did the STV in Taiwan where I am resident of... Sounded like you would be required to get background check both taiwan and US.... But in the end they did not require one at all, which was convenient.