SeatGeek is an event ticket marketplace and aggregator that collects and sells tickets from third-party vendors, such as Fanxchange, UberSeat and AJ Tickets, kind of like Travelocity does for airfares. You’re sure to find excellent deals. Its app is fantastic. Hankering to see Washington Nationals right fielder Bryce Harper hit his 100th career home run on a recent homestand, I downloaded the app and found decent upper-deck seats for $7 — a great bargain, considering that such seats are normally about $21 through the Nationals’ website.