I think the garage sale bargain can be distinguished. In that case, a human owner knowingly offers a cheap price and parts with the priceless item at that price. Here, on the other hand, I don't believe that any human at UA (aside from perhaps malicious employee(s)) intended to sell these tickets so cheaply. The tickets were sold by a machine, and humans discovered the pricing after the fact. It is not the same situation at all, IMHO.
(Note that I don't have a dog in this fight, either.)
But UA installed the automated ticketing system. UA had the option of having people check the validity of every ticket issued but they CHOSE not to. Shouldn't they be responsable for that choice?
I know I am responsable for my choices, why not UA?