Overstock claims they are selling about $1 million worth of goods per day. And they are currently converting those bitcoins to dollars. Essentially a million dollars worth of bitcoins is being sold every day. This is not miners mining and being Bitcoin supporters so they are more likely to hold than sell, but certain selling. At the moment $1 million would send the price down $30. It may not go down that far completely due to other adoption but it is a constant drain.
Every Bitcoin they sell has to have been bought or mined by someone. So every single Bitcoin they add to supply is a Bitcoin added to demand.There might be a tiny bit of short-term downward pressure as people who hold Bitcoins but don't really want to hold them take advantage of an opportunity to easily monetize them. But this is more than made up for by the increased value Bitcoins will have because they can be more easily spent.
