It depends on what type of airgun it is and what state you want to sell it in/ship it to.
Certain states regulate air guns diferently. For example, a handgun with a rifled barrel is considered a firearm in Michigan. Only smooth bore handguns designed to fire BB's only can be sold without a purchase permit.
AFIK, NJ, MA, MI, NY, DE, and IL have regulations concerning the shipping of air guns into the state. Check with the local laws before shipping to any of the states mentioned.