Do you mean buying one that is already making money?
Or do you mean buying one to start from scratch?
If so, I would not recommend buying one.
This is what I did to get approved really fast:
1) I told them I am an established business in my country who is making a lot of sales and who wants to expand my business using Merch.
2) Tell them all your designs are your own creation, they are of high quality and very unique.
3) Make them feel like they need you, not vice versa.
4) Write your application in a formal way without any grammar mistakes. First a greeting, then you have to state your purpose and as of last, end with a nice closing.