Buying wholesale sometimes is connected to having a sales tax number. However, each state has different rules about what can be purchased without paying sales tax on it even if it is wholesale. Some wholesalers require you to fill out a tax exempt form, which may/may not include your sales tax number. So, I guess you have to find out about sales tax within your state, then contact the wholesalers and see what they require (minimum order, letterhead, etc.) Or you may want to go to the wholesaler first.
You'll probably need a sales tax number when you are selling, especially if you are selling plants or cut flowers, so might as well get that started. Some states charge sales tax on food items also. You may also need a nursery license if you sell plants, a pesticide license if you use them, liability insurance for the farmers market (you may be able to add it as a rider to your house insurance if you don't have farm insurance), and any number of other things.
Someone from your county cooperative extension office may be able to help you wade through all of this, and they aren't connected with the enforcement of it, so it is "safe" to inquire.
Good luck with the new business.
Ann

Thank you anniew. I knew about some of the things you have mentioned and not about others. I appreciate the education. I never even considered the possibility of a pesticide license. I'll give our county cooperative extension office a call.

Thanks again,
Magret