There is evidence that Coinbase is selling on Bitstamp

[RSchexnayder]

The trading charts on BitcoinWisdom indicate Coinbase is disposing of its bitcoins on Bitstamp. I reached this conclusion by comparing the trading charts of Coinbase with those of Bitstamp, BTC-e, Mt.Gox, BTCChina, and OKCoin. It really does not matter which one you use from the 6 hour chart to the more frequent ones even down to the 1 minute graphs. However, it is not as obvious on the 1 and 3 minute charts as it is on the longer ones. There is no reason to use the 1 week to 12 hours charts because the Coinbase history does not go back far enough on the website. I load the charts of all six into browser windows so I can flip back and forth quickly between them to compare the patterns.
You can also tell this by comparing the actual trades on the BitcoinWisdom webpages on the two. For instance, at this time both exchanges are showing transactions at $824.75.
This explains why the price shown on The Genesis Block for Bitstamp is usually slightly lower than for BTC-e and BTCChina as explained further below.
The price charts on all of the above exchanges are quite similar indicating that there are active entities engaged in arbitrage between all of them as one would expect given the collective worth of the bitcoins being exchanged. Loading the six charts with the same time spacing into 6 browser windows as described above gives a real clear picture of the price discovery process between these leading exchanges. 
As any knowledgeable Bitcoiners is aware, Coinbase has to exchange bitcoins for fiat, so that merchants are not exposed to currency risk in an online transaction. They also have to be able to offload them somewhere, so they do not accept the risk either. As long as this pattern persists, bitcoins buyers should consider buying on Bitstamp and selling somewhere else. Hopefully, the competitors of Coinbase are using different exchanges for offloading.   
The only evidence that I have found so far that someone else has noticed this correlation between Coinbase and Bitstamp is found at this link to the Online Casino Archives, which states that both exchanges were trading at $814 at the time. If there is evidence that someone else has discovered this relationship before, please let me know. 

[skivrmt]

https://bitcointalk.org/index.php?topic=392422.0

Just discussed less than a week ago.  It's widely known that most likely they use them as an exchange.  A lot of people have done research on this subject already.  Coinbase of course takes a bit of a vig (mark-up) plus there fees on each buy or sell.

[aminorex]

not just likely.  this is clearly explained on the coinbase website.   all buys go through bitstamp as well.