By Mike GrassoPurchasing airline miles directly from the airline is often a terrible method of accruing miles, both economically and practically. Some exceptions are noted later. First, the airlines usually price these miles at a premium, valued far higher than earning miles by most other means (flying, car rentals, credit cards, etc.)
For example, United Airlines will sell you 10,000 miles for $303 dollars. That $300 bucks could easily fly you to New York City for the weekend, and you'd earn 10,000 miles in the process. Therefore, there is no direct value added when buying miles from the airline.
Second, redeeming miles into a capacity controlled frequent flier program means the airline will limit your redemption opportunities on days, flights or routes it chooses. If it's a popular route, high-season, or holiday, you will have a tough time finding availability. They would rather sell expensive tickets than fill the plane up with people redeeming miles. And guess what? You are not the only person trying to redeem miles on a flight. Many people are competing for what is often limited overall availability. Thus, buying premium-priced miles for use on low-value flights with the added uncertainly seats will even be offered by the airline, isn't very practical.
But, buying airline miles can be situationally helpful. If you are 1000 miles short of 50,000 - enough for a free economy ticket to Europe, you may want to fork up the $50-$60 cost. In this case, call the airline first - check for award redemption availability before buying miles.
Another example is maybe you are 5,000 miles short of 90,000 - enough to fly Business Class on Singapore Airlines to Asia. The true value of that business class ticket is probably $6,000 or more. Paying $150 to cover your miles shortfall does seem tempting. Again, check for availability of award seats.
Lastly, as if they airline doesn't already have a strong-hold on these frequent flier programs already; buying miles is not a real-time exchange. United Airlines says it can take up to 48 hours to post to your account, American Airlines up to 72 hours. So while you are waiting for those purchased miles to post to your account, award availability may vanish from under your eyes. If you are in a situation like this, call the airline first, explain your scenario. They may do a courtesy hold if you provide them with your confirmation number of your miles purchase.