Don't blame valve too much, blame all the morons who apparently hand out their account info to anyone who asks, or click on links from some shady guy who just added them 5 minutes ago and promised free tf2keys for logging in to his website, or any number of other completely ridiculous scams.
The number of people who had their inventory cleaned out because they are complete idiots is unreal. It's a lot of work for valve to fix it, and unfortunately they can't really just say "sorry you're a moron you deserve it". So they had to do something to protect people from themselves - and unfortunately you just happened to get screwed over by it more than most.
Heh. I am constantly more and more glad I don't use the Steam market for anything but free funds from sold game cards.
Ha, I can't even sell all these stupid sale cards because I've "recently funded my steam wallet using paypal or an unverified payment method that has had a recent increase in use". I'm not even sure WTF that means, I used a credit card that I'm relatively sure I've used on steam before (though it was a few years ago). I'm also not sure why it stops me from selling things either, I guess they are just too lazy to split buying and selling and just put a flat market ban on because it's easier? There's probably a trade ban to go with it, but I don't know for sure since I've not tried to trade anything recently.
It ends a couple of hours before the cards poof so I guess I just have to hope I'm on at the time so I can dump them and get my 50 cents or whatever