With ATV parts, if you are expecting to sell genuine OEM parts for major manufacturers (Yamaha, Kawasaki, Honda, etc.), then you probably would need to be a franchise dealer for that manufacturer.
You don't need to have a franchise to sell aftermarket parts, but you'll need to go thru an aftermarket distributor or wholesaler. Here is an example of an aftermarket wholesaler: http://www.northernwholesale.com/
I suggest contacting one or two wholesalers and asking a lot of questions to learn the lay of the land. Sounds like you've got some due diligence to do.