Rather risky. If the airline catches on that someone sold you a ticket with miles from their account they could deny you boarding and leave you stranded somewhere. Take a look at this thread where a flyer was denied boarding on a trip to Australia because the airline suspected fraud:
http://www.flyertalk.com/forum/unite...ket-fraud.html

And another intereresting one:
Consolidated "My MileagePlus Account is Being Audited" Thread [Merged]