a 1/2bbl keg is worth about $20-30 in scrap metal. they cost about $150-200 new if you are buying them several dozen or several hundred at a time (or $250-350 if buying one at a time). a non-private seller offering used kegs for $100-150 is a good deal.
if you look hard enough you can find private sellers on ebay/craigslist who dont know what the fair value of a keg is and offer it for less. but more than likely these people simply kept their kegs and forfeited the deposit.
the $50 deposit that the liquer store charges you in no way covers the replacement cost of a keg, and paying that $50 charge does not mean you own the keg. more than likely, if you find a keg on craigslist for $50, its stolen property. its hard to prove if it is or is not stolen though.