OP, I might be misunderstanding you, or you might be conflating two or even three different types of operations.

1. There are online travel agencies (OTAs) such as Expedia, Vayama, etc. that sometimes can offer discounts (usually modest) from what you'd find at airline sites. A problem with these is that many can be difficult or impossible to deal with when you want to cancel or make changes that would be relatively easy if you'd bought directly from an airline. I had a horrible experience with Expedia along these lines, for instance. I've very occasionally used Vayama without problems, but never had to make changes with them. Then again, some OTAs are simply unreliable when it comes to just purchasing tickets.

If you go to Kayak.com or modomo.com and search for prices for a given trip, sometimes such discount online travel agencies pop up as options. But I believe the accepted wisdom here at FT is that travelers are generally best off purchasing directly from airlines.

The second operation comprises experts who know how to find award tickets and for a fee ($100? $200? I can't say because I've never used them) will find them (if they're available, which is a big if) for customers who have the miles but not the time, patience or knowledge to look themselves. There's a thread here at FT that lists and discusses such services, but offhand I don't know where it is.

The third is OTAs that will sell you tickets using miles that someone else has sold to them. Airlines bar such third party transactions, so this is a very risky endeavor for the buyer.

Hope this helps. PM me if you have more questions.