aluminium casks do wear out as they are lined, but the more common stainless casks last virtually forever, so I would ask that people don't use 'old casks\kegs unless bought from the owner. They are very expensive to buy and a continual headache for brewers as they are easily stolen from pubs etc.
Ceratinly all our casks are stamped with our name and serial number and the coloured bands are used as easy visual reference of ownership.
that's fair enough, stainless casks do come for sale but are usually snapped up pretty quickly by brewers, albeit they command a fair portion of the price of new ones (about £70.00 + vat new for a firkin (9 gallon). Incidentally a barrel is a 36 gallon cask in brewing terms, and kegs are for gas dispensed beer.