There are a number of federal regulations with regard to the sale of firearms. So if you want to sell guns across state lines, you will need to have a federal license. Even dealers that have a license are subject to restrictions with regard to the following: - Selling automatic weapons. - Selling firearms to someone who is under the legal purchasing age. - Selling guns to a felon. - Selling guns to an undocumented immigrant. - Selling destructive devices. One thing you should know about these crimes is that there isn’t just a “knowing” statute. Sellers have to make sure they don’t violate these laws, or they can face the consequences.