-Here and other forums
-Craigslist for your and surrounding areas
-Any LFS that will let you - Reefs Aquatics was talking about a bulletin board for customer used goods and maybe this could be the thing to get it started?
And not exactly hobby focused, but grocery stores and places like Gander Mountain that have customer boards at the entrance. To be honest though, your best bet is going to be on here or Craigslist, and if it doesn't sell, drop the price until it does. Good rule of thumb for a standard size tank is $1 a gallon and 50 to 75% of equipment costs that is in excellent condition if there is any. Start a little higher than you need to be to give a little room for a discount if need be and drop the price every 10 days or so until it goes. Most in this hobby are much more active in the colder months, so if you're not in a rush you just need to wait until the right person finds your tank. You can repost an ad on Craigslist every 45 days.
Good luck...