Is buying and selling bitcoin in large volumes possible, or wise?
It is possible. You can go to Mtgox if you like, but of course you can make an offer in BITCOIN-OTC, "outside the market". If someone agrees your offer, this trade would't impact on Mtgox and similar markets.

It may be wise. You only need to be careful with people you agree to trade, using the "trust database" to see people's reputation, so you decrease the probability of getting ripped.