Every person will value them differently, but it's a shame that it is not easier to buy/sell/trade them. For example, I value my hundreds of thousands of UA miles for very little because they never want to redeem them. However, I value SWAir tickets more highly because they are so flexible.

If you want to liquidate at a "market price", here's the skinny from www.awardtraveler.com, a FF mile broker:

"If you may want to sell air miles Award Traveler generally pays between 1.0 and 1.6 cents per mile.
For example, depending upon the airline, Award Traveler may offer you $520 for 40,000 miles; $1260 for 90,000 miles; $2250 for 150,000 miles; and $3200 for 200,000 miles.
Award Traveler also pays $240 RT for Southwest Airlines Rapid Rewards Award Tickets with at least 3 months remaining before expiration."

I would be interested in someone starting a separate posting on their experiences with mileage brokers. Ethics and "airline rules" aside, I'm looking to dump a pile of miles that the monopolists won't let me otherwise use (and as MileKing stated, use the cash to buy heavily discounted airfare).

Another side point: with SWAir, 4 RTs purchased online will score a free ticket. RTs can be had on many routes for <$100.