It is definitely a bad idea. Any ticket for someone age 10 or older whuiich has been used is tied to a person by a finger scan. If you go to the locked sticky Everything About WDW Tickets in this Forum, especially Post #14, you can get a lot more information. An easy way to get to the sticky is by clicking on the link in my signature.
If you show up at a turnstile and an "identification fails" indication comes up and you still fail when trying again, the CM at the turnstile can ask some questions to determine if you are the legitimate owner. If you cannot answer them the best that can happen at this point is the tickets will be confiscated and canceled and you will be asked to go to a ticket window to buy new tickets. The worst that can happen is Security will get involved.
But you will be out the tickets, the original owner will not be able to get them replaced, and you will have to come up with money on the spot to buy legitimate tickets.