There are stories of fake ticket scammers all the time. It's illegal and happens a lot. While I don't doubt there are many honest people with honest tickets to sell, there's no way to know for sure. I mean, I'm sure many "reputable" sellers are using illegal tickets.
Plus, what also can happen is that an "honest" person may purchase a multi day ticket, use only one day, and try to sell off the rest--not knowing (although many fully do know) that the remaining days are tied to the finger scan of the original user, and won't work on anyone else.
If mean, if you're happy either risking fake tickets (and getting called out on it while at the turnstile does not look fun--I've seen it happen) and/or supporting a criminal activity, then go for it.