The annual limits on buying miles from the airlines are far too low for this. One reason is that they don't want people to start using that as a cheap way to buy tickets that would otherwise cost a lot of money.

Buying miles from brokers, other individuals, eBay, craigslist, etc., is against the terms and conditions of every frequent flyer program. The airlines police this sort of activity carefully, cancel the accounts of the sellers, and invalidate the purchased tickets - forcing travelers to pay last-minute fares or stay home (or not get home). If they didn't catch a lot of people, they wouldn't continue to pay for their "revenue protection" departments. While you may hear from some people who have gotten away with purchasing miles or award tickets, it is emphatically NOT recommended! (Folks on eBay, etc., who claim to get around the rules by selling you a plain white envelope and giving you the ticket as a present accomplish nothing via this sham. Ask any lawyer about "form versus substance.")