Tony Avent from Plant Delights Nursery has a great book on starting a nursery, a must read.

I didn't plant on making much money with mine (it wasn't mail order, tho) but with the taxes, license for business, and license for nursery, and other costs, I ended up losing money.

I'd say you have a have a great shipping method, lots of plants get damaged in the mail, and you'll have to replace them.

Tell us more about what kind of a nursery you want, will you be the only employee? Will this be your only job? What area do you live in? What kind of plants do you want to sell?