For things like tanks I have found a great way get a "fair" price is to take them to a dive site (Ginnie would be a great choice) with lots of divers and set them out when you (or your friends) are rigging up or a hanging out.
Pick up a $1 For Sale sign and see what kind of response you get. Lots of Intro divers looking to upgrade or just other divers wanting a spare set would get chance to look at the goods.
I would be totally surprised if you couldn't' sell a lot of the gear at the upcoming Holiday weekend - note you might have to go back to town to complete the transaction (bank machine) or get to a lap top for paypal.

