The only program I’ve ever been inclined to buy miles from is British Midland, because with their cash & points award chart and unfiltered access to Star Alliance first class awards those miles have an especially high value. Members can purchase up to 20,000 miles during a 12-month preiod, and until September 30th they’re offering a 10% bonus for doing so.
The current signup bonus for new members of the program is 3,000 miles.
So a new member maxing out on the purchase transaction with bonus will have 25,000 miles at a cost of ~ US$480, which along with a roughly US$340 co-pay, is enough for a one-way international three-cabin first trip between Japan, China, or South Korea and Hawaii.
Add in 16 Hertz rentals at 3400 miles apiece and you can fly international first class roundtrip from the US to Asia or Australia using the cash and points chart (co-pay: US$540, total cost probably around $1800 plus taxes, a lot of money to be sure but really quite cheap for tickets with retail prices of $10,000 to $15,000).
British Midland is also the only program outside of North America with a regular status match program. Their Gold status is especially valuable.