https://gatecoin.com/blog/2015/10/gatecoin-launches-rep-trading/

...How did we achieve that?
We pooled some of our largest customers to buy REP through our platform, enabling them to list their tokens on our exchange.

As a result, our customers are now the only ones in the world to be allowed to buy and sell REP!...

http://coincurrencynews.com/2016/02/26/augur-rep-only-sold-on-gatecoin-exchange/

...It’s true, so far, the only exchange to be selling the Augur coin is Gatecoin Exchange. However, there have been rumours that the coin could make its way to other exchanges in the future. While at this time, the prices have been climbing, it is tough to say whether they will hold, or what this coin will exactly do. So far, it is being likened to a “reputation” coin, hence the (REP) ticker. That means that if you are getting a bad rep on the internet you can pay some REP and increase your popularity?...

https://augur.zendesk.com/hc/en-us/articles/205129442-When-will-Augur-s-REP-tokens-go-live-for-trading-What-about-exchanges-already-trading-them-like-Gatecoin-