This can be a source of cheap air tickets. However, you'll be traveling under someone else's name, and the airlines frown on this practice. They may also terminate your membership in their frequent flyer program. Their contract is with the person named on the ticket, not with you. If the tickets are actually stolen tickets, they will definitely catch you.
Many airlines offer a bounty to employees who confiscate such tickets.
But more are checking IDs on randomly selected tickets in an effort to discourage this practice. All carriers reserve the right to require identification and to refuse transportation if identification cannot be provided. Boarding may be denied and ticket confiscated if passenger's identification does not match the name on the ticket. If the person named on the ticket checks in for you at the baggage check-in counter, you'll probably run into trouble on the return trip. Plus, many airlines now check IDs on boarding at the gate, to discourage this practice.