Never had a problem, even when I've been given 'print at home' tickets and used those. I felt a little sketchy about holding something out of someone's printer, but got let right in.
Essentially, Stubhub has to take care of it if you're given bad tickets. They have offices near the stadiums of teams/concert areas in most major cities, so worst case, you'd have to walk down the street and pick up something there. It's never come up with me, so not entirely sure how it works if you're denied at the gate. Might end up missing a little bit of the game/concert if you were running late, but they say they'll get you in regardless, so it should work out. It's a reputable site. Only bought from them a couple times (3 maybe?), but have sold probably 20k worth of tickets there in the past couple years, so familiar with the way it works.
Real reason that it won't happen is that they penalize the crap out of you as a seller if you do that. You lose the money you were paid for the ticket, and you're charged a huge fee to find the 'replacement tickets' for the event. Plus, you'd lose your account and any possibility of selling more of your tickets there, so pretty good reason to not screw with the system. Sellers have credit cards on file, so they can't avoid paying the fees, either.
Regardless, i only sell the venue-printed tickets, and state that on my ticket listings. Makes people more comfortable holding the original ticket rather than a printed email, and just saying that makes people happier about buying MY listing instead of someone else's that didn't specify.
