If you are on the hunt for a used cylinder, here are a few things you might want to look out for to save yourself from purchasing a used cylinder only to find it turns out to be useless to you.
To be on the safe side I wouldn't buy a cylinder any older than 15 years. If it is not mentioned, ask the seller the year of manufacture which will be stamped on the cylinder. And when did it have it's last hydro test and inspection. Cylinders must be tested annually so if the cylinder is out of date or nearly due for test, then call your local dive shop and find out what it will cost for you to get it tested because you will have to factor that into the price.
Have a look at the valve and make sure it is fitted with a burst disk (it is a requirement that valves are fitted with a bust disk and if it doesn't have on you won't be able to get it filled). And make sure the valve is compatible with your regulator (din or yoke).
Find out the air pressure in the cylinder. You should never completely drain a cylinder because it allows water, particular and contaminant to enter through the valve. So if the cylinder is completely empty then there is a chance it will have problems passing an inspection. And the internals should be cleaned by a qualified technician prior to use. If the cylinder is fitted with a boot then make sure the surface under the boot is ok. Boots tend to hold salt water which can damage the surface of the cylinder over time leading to problems when getting a visual inspection.
Buying a used cylinder can be an affordable way to own your own cylinder... but I do see a lot of duds for sale online. If your shopping for a used cylinder, consider the above information and you should be able to rule out the duds and identify keepers.
Cylinder date of manufacture will look something like this and will have the month, followed by a manufacturer symbol, followed by the year.
A burst disk will generally be fitted with a hex head bolt keeping it in place.