The ignorance, it's untrue.
They built a system that should have failsafes, to allow trading of certain digital goods. Good restricted to within their system. The end user or the buyer cannot, will not, ever, in this life or the next, this universe or a parallel universe know if a digital item is stolen or not. All you see is that a game is for sale or trade, there aren't physical markings to check, pictures, serial numbers (not to be confused with serial keys) and such.
So, you say how can Valve know that he is not deliberately dealing in these goods? That's a good question, how can they? Why would they ban and not simple use support staff to investigate and enquire.
Why is the go to method to simply just ban that person and forget about it, innocent or not? Even when said person tries to contact them and clear up the situation.
Sorry but, some cases are quite cut and dry and corporations simply have the ability to not care for the odd loss of customer, this is a problem because people like you and I could be next.
(It's late, probably lots of grammar mistakes)