You need to know about the RISK in buying these types of tickets. It's a frequent topic of discussion here on FT.

Biggest risk: if you are buying someone's miles to fly across the world, and the airline catches on, for whatever reason, they could deny you boarding and require you to buy a full fare last minute ticket to board. You have no way to know if they are monitoring the account the miles may be coming from, and if an airline agent catches you off guard, you may very well give yourself away about the "great deal" you got on your ticket. There is suubstantial risk in buying miles that belong to someone you do not know, and the airlines do not need to honor these tickets if they suspect they were sold.