anybody ever 'sell' their wireless contract?

[Kiva]

Ok, called to have my sister-in-law dropped off our ATT bill and turns out she's committed until Dec. It was $90 to cancel her. The rep advised that I look into 'selling' the plan to somebody else. He said that it would be somewhat of a commodity as it is upgrade eligible and expires in 6 months.

Anybody ever done this? I see a couple of websites that specialize in this, but they charge like $20.

Any experiences out there?

kiva