In principle I have to agree with you, but economists by training are trained to look at free markets. This market is muddied by the fact that selling miles almost always violates program terms. It exposes both buyer and seller to risks that include confiscation of miles, closing either or both accounts, and the buyer showing up at the airport with a worthless ticket and being told "pay the last-minute fare or don't fly." This reduces what informed people (let's not get into uninformed eBay bidders; that's a separate topic) are willing to pay for miles to less than they're "worth."

Airlines, by contrast, knowing that they have a legal monopoly on selling miles in their own programs, charge a premium price above what they're "worth." They get some buyers, but not many (at least not from here).

If eBay selling prices are below their value and the airlines charge more than their value, one could make a case that their real value must be somewhere in between. That would be in the frequently-cited range of 1¢ to 2¢ per mile, well below what one would get from the selling price of many "high-value" awards. In fact, one reason for limits on how many miles one can buy in a year is to keep people from getting things like trans-oceanic F/J tickets for a fraction of their price by buying miles and claiming an award.