Your best bet would be to advertise in the free classifieds available on this and some other
forums. If I were doing it, I would put a
price on the
regulator and give the cylinders away with it for free. You will not want to, nor will you likely be allowed to, ship the cylinders, so you will have to find a buyer who can make a local pickup. The new owner is just going to exchange the cylinders for new ones with
fuel -- they will not have to worry about the
certification of the cylinders themselves, the
fuel distributor does that (and in my experience their dealers just do not bother to look at the
current condition of the cylinders). There are still
boats out there that use CNG for
cooking fuel (mine is one), and there are still some places to get it, but I don't think you are going to find any potential buyers among new users...
Good Luck!