It varies from airline to airline. While upgrades are less generally useful than the number of miles it would take to upgrade the same trip, you would expect to pay correspondingly less for them. If you could have obtained the requisite number of miles for the same amount and would then have had the flexibility to use those miles in any way you wanted, the miles would have been a better deal.

That said, you should also be aware that selling miles is against the rules of every frequent flyer program I'm aware of, and selling upgrades or other certificates is against the rules of most. Those caught violating the rules may not have done anything "illegal" - in California, they may run up against a statute prohibiting use of a ticket for transportation in violation of its terms and conditions - but in any case they are subject to a variety of airline-imposed penalties. These include making a ticket obtained or upgraded with purchased miles or certificates invalid, forcing a traveler to buy another at full walk-up fare or not fly, to say nothing of canceling the FF account of buyer and seller with loss of all miles therein. Many airlines have departments that monitor Ebay and passengers' travel patterns to identify such sales. They don't do a perfect job, but they catch quite a few people.