Example with airline miles:

This summer I want to go to Europe. Thats 60K AA miles roundtrip in coach.

So..

IF I USE MILES

Beginning balance: 60K
Ending balance: 0K

IF I SELL MILES/PURCHASE TICKET

Beginning balance: 60k
Ending balance: 15K (Around that depending on segments...)

By purchasing I have more miles in the end and spend the same or just a bit more money... here's how it works:

The thing is you need to know someone that you can trust to actually use the ticket/give you the money.

So, if they are looking for a $1,200 ticket, you sell it to them for $1,100 and use your miles to purchase it under their name.

Then purchase your ticket (pay any difference if necessary).