A lot of the cheaper ones on eBay seem to come from BestBuy. I'm assuming they're legit. It looks like BestBuy's strategy is to start the bidding low and hope that it goes high enough most of the time to cover their dealer cost.
I bought my copy from SF for $249, boxed. There was an ad in Computer Videomaker with a special internet link that set the price at $249. This price is usually associated with the "competitive upgrade offer" but at no time during the check out process did SF ask me or require me to have already purchased a competitor's product (i.e. Premiere). I think the ad showed up in the January edition of Computer Videomaker, haven't seen it since.
Mine came in a retail box. The shrinkwrap plastic wasn't on it, but it had 2 cd's, 2 manuals (getting started and tutorial), serial numbers and I think a postcard to register.
I was able to register it without any problem, so it must have been legit.
For those interested in the Academic versions...from what I've been told, these versions are not licensed for professional use. Meaning if you want to make $$$ with Vegas Video 3, you are not supposed to be using the academic version. I don't understand all of the red tape behind this, but was given this information from Academic Superstore.