If you want to resell freon from your home air conditioner, it is best to check the legal rules in your state first. An easy way to do this would be to contact a HVAC contractor in your local phone book.
Sell to a Contractor
The best way to sell freon would be directly to the HVAC contractor. Many of them may be willing to buy it from you, as they will be able to see if it has been used or not. It is also an easy way to sell it on your initial call to check the state laws.
In some situations, you may even be able to sell it back to the manufacturer.
Selling the Freon
If you are able to sell your leftover freon, make sure you specify the amount, type and unit it goes with, so that it is easily found by potential sellers.
It is not a good idea to try to sell partially used containers, as it could cause messes and legal issues if polluted in some way.
Also, make sure to sell it to someone who is certified to use freon.