Anything is cheaper per unit when you order in large quantities and ATV tires are no different. Tire shops and other stores where you usually buy your ATV tires will in most cases agree to give you a lot better price per unit if you order multiple sets of tires at once. It is a lot more pleasant for the merchant to sell off for example 10 sets at once to one customer instead of selling them separately to 10 different customers and time is money. Every merchant will have their own policy on wholesale of their ATV tires, some will encourage it and maybe even have ready deals while others you will have to personally contact to agree on prices.
Now you might think that wholesale is only for the stores and dealers and not for the individual ATV owner and you are right, but the fact is that anyone can order in bulk and if you can gather a group of individuals you will be able to order under wholesale prices and still use individually. Gather with your fellow riders and see who needs what and before you know it you will have a large order totaling something that could be bought for wholesale prices. If you want to take it one step further and you have a basic understanding of the market and how many potential customers you have it might be profitable for you to order ATV tires in wholesale and then sell off to others around you.