Here are some examples that could happen just off the top of my head. Lets say if the seller has posted his/her tickets on both StubHub and Vivid Seats. Lets say those tickets were sold on StubHub first though minutes later they then were bought on Vivid Seats before the seller was able to remove them. I had this happen to me with Raptors tickets last winter, though Vivid Seats actually called my cell phone and said the seats I bought were no longer available because the seller already sold them previously. Also, if the seller has those seats listed on StubHub, though is also listing them on lets say Kijiji and finds a buyer who meets up and pays cash, and the seller forgets or doesn't remove those seats from StubHub, then yeah you could get screwed if that buyer from Kijijji scans his/her ticket first to enter the venue.
At least with StubHub or Vivid Seats, you have 100% protection guarantee when buying, so if you don't get into the venue, Stubhub will try to find you other similar seats or give you a refund. Thats the key rather than buying off someone from Kijijji or a scalper outside the ballpark.