It depends on how the guy holding the garage sale got them. Kegs are the property of the brewery, and they don't sell them to people. There are some wholesale recyclers who buy them, like Sabco, and either recondition and sell them back to breweries or convert them for other uses. If you see a used keg with an old brewery name on it, you can be almost certain it was obtained illegally, by someone paying a deposit and never returning the keg. If illegally obtained, you can't legally buy it. Just like you can't buy a car if the seller doesn't have clear title, you are not in the clear buying something legally unsalable at a garage sale, even if you did so in good faith; you would be receiving stolen property. Breweries lose enormous amounts of money through kegs walking away, and we all pay for it.