I would never advise buying tickets on the street so, to me, the only question is that if someone shows up with a fake ticket, the house has to deal with it. It is easy enough in the theatre to figure out who has the legit tickets, and who should have the police called for. It is a bigger issue at venues where you don't have to be seated at any point (e.g., baseball games), or general admission events, etc. where you may never find the scoundrel (or unlucky innocent). Regardless, the legit ticket holder can be seated.