It is completely legal for you to produce liqueurs using the processes of infusion and blending, but illegal to involve or use the process of distillation without proper permits and licenses.
Alcohol bases used to make your liqueurs, 190° proof grain alcohol, vodka, brandy, etc., must be purchased legally. Alcohol can be transported between states, even if it is not sold or illegal in your state. Check with your state liquor board for quantity limits.
It is illegal to sell or offer your home-made liqueurs for sale. However, it is perfectly legal to produce liqueurs for you and your friends’ personal consumption. They make great gifts! Whatever you do, do not get money involved in the consumption or distribution of the liqueurs! The Feds want their taxes!?
If a name has a ™ or ® beside it, that means it is a trademark or registered trademark, respectively, and is considered proprietary information of that company. The name should not be used to label your home-made liqueurs.
If you are a creative naturalist interested in making new and unique liqueurs, here is something to think about...
For well over 100 years, Wormwood, a common herb, had been used as the major flavoring ingredient in the very popular Absinthe liqueur. Drinking Absinthe in large doses caused irreversible neurological damage due to a strong narcotic, thujone [controversial evidence], found in Wormwood. Absinthe liqueur was finally illegalized for sale and manufacture in many countries, including the United States, on March 16, 1915. It was invented in the late 1700’s.
Update: Absinthe has recently been legalized in the USA; with regulation stating that the liqueur must not contain more than X amount of thujone.
If you are going to experiment with wild plants or anything for that matter, make sure you know what you are dealing with. The sky is the limit, and who knows, maybe you will come up with the markets next-best selling liqueur.