Based on panicked posts and inevitable memes, it looks like the trouble started a couple hours ago at around 5pm GMT. Down Detector shows a similar timeline of user reports flowing in. As for right now, success seems mixed. Alice0 helpfully attempted a purchase and was given the message “There seems to have been an error initializing or updating your transaction. Please wait a minute and try again or contact support for assistance.” I, on the other hand, am now the proud new owner of $0.99 game Otherside. The Halloween Sale has ended now, unfortunately, but you can still try your luck with getting a purchase through on everything else. I’ve reached out to Valve to see if they’ve identified the cause for this truly spooky development (whether it was Halloween-sale related or not) and will update to let you lot know what they say.