Scalpers serve a purpose for those who don't want to spend the time buying or selling tickets for a price they should be. They don't care about getting lowballed on their tickets or overpaying a bit for convenience. However I rarely deal with them, as there are always good prices on ISU tickets (buying or selling) if you put a little work into it.
I always low ball scalpers and pit them against each other, it almost always works like a dream. Sold-out events are a little different, but I have also had some of my most successful scalping endeavors at sold-out events.
Also, wait until as close to game time as possible, they start to sweat. Their tickets are almost worthless 5 minutes after any event starts and they know it.
Treat them like **** and don't be afraid to walk away. These guys do this as a serious hustle and can spot a mark a mile away. If you don't know what you are doing or aren't 100% comfortable with making cash deals with strangers that have a certain amount of risk involved, just use an app. It's 2019, your phone can literally be your ticket.