Hi Nick,
There are many ways to list/sell it. The first place I’d list it is right here at SMF. There’s a Classified ad subforum here where members post bbq related for-sale items all the time. And if you’re a member of any other bbq related forums, chances are they have “for sale” sections too. Lots of folks who are looking to buy a used smoker will check these listings first.
And of course there’s always Craigslist. Opinions vary, but I’ve always had pretty good luck selling bbq gear on CL.
For best results with forums and CL listings, take a few good pics of the smoker and post those with your listing...and give specific information about your smoker (size, age, who built it, whether your asking price is negotiable, your location, whether you’re willing to ship it to other parts of the country, etc.).
And then, depending on where you live, there could be some locally specific seller options...like local auctions, local newspaper classifieds, consignment shops, flea markets, etc.
Good luck!
Red