Tread very carefully here.

As mentioned by another poster, most airlines have personnel that monitor sites such as Craigslist, ebay, etc. If caught, it's very likely that the airline may close your frequent flier account and revoke all miles.

There are many stories here on FlyerTalk of this happening.

A friend of mine was selling system wide upgrades from American on ebay. American was able to identify that he was the seller and closed his account. He lost a substantial amount of miles.

FYI, this is what is stated in Frontier's Early Return membership terms and conditions.

"The sale or barter by Members of any such mileage, awards or benefits is expressly prohibited. Any mileage, awards or benefits transferred, assigned or sold in violation of the Program terms and conditions will result in the confiscation of all miles held by the Member and closure of the Member's account. In addition, the Member may be subject to other penalties including, but not limited to, the value of transportation provided as a result of member's fraudulent conduct. Mileage, benefits or awards earned or granted under the Program may not be transferred or assigned except in the case of a member's death or online using Frontier's Transfer Miles tool. Miles may NOT be transferred via phone or email request to the EarlyReturnsŪ Service Center. Upon a member's death, all current available mileage may be transferred to the member's beneficiary after receipt of appropriate documentation such as a death certificate and proof of beneficiary status."