A friend has a t-shirt business online (www.honeville.com) and seems to be doing fairly well.  At one time he was the largest t-shirt shop on ebay.  I think he is trying to move in other directions because of some changes ebay has made.  He doesn't have a "farm" section to his web site, although he does have a "country" section with things like this.

My first thought was like others, farmers make up a very small percentage of the population.  But your market is several times the number of farmers.  Think of all the John Deere branded apparel sold to non-farmers.

I think that is one area to be careful of ... licensing.  Be sure and have the license to use logos, trademarks, brand names, etc before you use them.

The key to your success is marketing.  Finding a way to get your name and your products in front of potential customers.  Can you find success?  As Gramps told me a long time ago "There's a lot of ways of making money".