Last year, Valve started dropping a hammer on gambling sites selling Counter-Strike items. This week, the same process has begun for Team Fortress 2.
In a short statement released online, Valve say that following the ban on CS stuff in 2016, "some gambling web sites [have] started leveraging TF2 items. Today we began the process of blocking TF2 gambling accounts as well. We recommend you don't trade with these sites."
Those sites, like the CS ones, are shady af. It's a solid recommendation.