Here's another angle on this whole issue according to a family friend I talked to over the Thanksgiving holiday.

The issue of coupons not being "valid if sold or bartered" stems not from a legal issue but from an accounting issue.

This argument says that the reason coupons and FF mileage programs are not transferable is because the airlines need to be able to argue to the IRS that the coupons, miles, and points they give away have no value (i.e. can't be sold). If the coupons could be sold then they would have financial value and would have to be booked as a liability on the the airlines books (they would have to be expensed).

The airlines don't want this expense to hit their Profit/Loss statement because it makes their Earnings Per Share go down.

The 500-mile certificates that UA does sale are transferable because UA sells them (at least some of them) and accordingly books them as a liability and an expense (just like standard tickets).

Are there any accountants out there that can shed light on this?