If you attempt to buy an item that costs more money than you currently own, it asks you to login to buy, even if you are already logged in. Hovering over an item that costs more than your current value shows it is attempting to bring you to titan.tf/login. Actually clicking the item brings you to titan.tf/home. I checked black market and the mystery boxes, and this does not seem to be a problem there.
https://imgur.com/a/jiXBoJM
First image shows the page without hovering over anything
Second image shows the page hovering over buying the Megalovania Dispenser theme.