There are plenty of scalpers outside the stadium and you definitely know who they are. They usually mark the tickets up 20% but feel free to negotiate with them.

Dealing with scaplers is always a no-go in my book. Because this is such a big stadium, you are always going to find a scalper or two hungry for some profit. All you need to do is be able to identify them (they ask you if you already have a ticket), and be sure to nicely walk away. Try and prevent any confrontations if possible, as it might end up badly for both parties!