Many topics, including this exact topic, have been discussed many times on this site. Before asking a question, you should try a site-specific google search, like this:
https://www.google.com/search?q=selling+heads+site%3Aadiforums.com
Here are a couple previous threads.
A couple suggestions, from a previous thread that I was not able to locate immediately, suggested to either:
a) sell as charcoal lighter fluid.
b) sell to a marine fuel company as an enhancement to “bunker” fuel.
Years ago, I contacted numerous marine fuel and alcohol recycling companies. Absolutely none of them had any interest whatsoever in buying heads. One of the few companies that bothered to reply told me they would haul the heads off for free. However, they wanted extensive documentation as to the purity, composition, etc.
Bottom line: as far as I am aware, no one will buy your bulk heads. You may be able to sell them in small quantities as charcoal lighter fluid, but to do so you’ll probably have to denature them, which will result in additional paperwork and proper registration in Permits Online.