Are miners currently a large enough portion of people selling bitcoins to significantly impact their price?
3600 bitcoins are mined every day. About 50,000 BTC are traded every day. If every bitcoin that was mined was sold on on an exchange, those sales would only account for about 7% of the total exchange volume.
The fact that mined bitcoins are a small part of exchange volume and that there is no mining cartel, leads me to believe that the actions of miners have no effect on the price of BTC.