On very hot events though, like One Direction 2 years ago. (Bought a pair of Center floor row 2 for $300, sold for $2750 per ticket).
Sports are usually not hot for the regular season for all sports except NFL. A lot of the cheap tickets you see on Stubhub during the regular season are from Season Seat holders and not Brokers.
Only when it comes to the NBA finals, NHL Finals, Superbowl, the tickets are grab-what-you-can, and are guaranteed major profit.
Usually Round 1/2 are not profitable enough for any sporting events unless it's like Heat vs Pacers Game 1, or OKC/LAC Game 1.
US Open Tennis is usually always a big profit no matter what day you buy and sell tickets for. Obviously the final day is the biggest profit, but even Day 1 cheap tix ($300 ea.) have fetched us $650 per. We got a single cheap ticket last year for the final day for $500 and sold it for about $1250