I signed up for Coinbase myself and was reluctant to use them initially because the fees are relatively high compared to the exchanges, but it's so easy to use that I'm willing to put up with a 1% fee. I was hoping to use bitstamp with their lower fees, but since I don't live in Europe the fees would actually be higher than Coinbase's fees overall since I'd have to pay wire transfer fees or SnapSwap's fees if I went the Ripple route.
My personal theory on why the price is lower on non-MtGox exchanges is because the volume is so much less. More volume tends to push up prices, so MtGox prices get pushed up and the other exchanges prices tend to stay lower.
It depends. If you are selling because you want to sell bitcoins and buy dollars you should be fine. If you make it your business to buy and sell for people and take a fee you are a broker and must register.
I've had nothing but good experiences with Coinbase, but obviously you need to make your own assessment of the risks of any business you hold money with.
That makes sense. Oh, I'm not complaining about Coinbase as a company. I used them to buy my first bitcoins and the process went very smoothly, the coins arrived when they said they would. I just like to minimize my fees when buying, that's my reason for being inclined to buy elsewhere. I'm looking at CampBX now, but I haven't decided for sure if I'll switch yet.