LouGroza is 100% correct on the tax issue.

FF miles are not taxable when received from FF programs (see IRS Announcement 2002-18). That reinforces the notion of you having no cost basis in them. Selling anything in which you have no cost basis produces taxable income.

Will the IRS show up at your door if they found out you sold a friend an award ticket for a few hundred dollars? Probably not (although you'd probably get taxed if the money your friend paid you showed up in a bank account audit). Will they show up at your door if you are listed as a mile seller with a mileage broker? Very likely.