I usually buy my bitcoins at Bitstamp.
Surprisingly, Bitcoins are trading there at about 10% lower than at MtGox, the most wellknown exchange. (So for selling I would consider MtGox rather than Bitstamp.) Furthermore, at Bitstamp, fees are lower and validation is less complicated than at MtGox.
I recommend to have more than one option for buying or selling bitcoins, because exchange sites are occasionally down.
Mt. Gox is becoming insolvent. They have no U.S. funds anymore, I would be careful. Withdrawals haven't worked for over a month.