I would recommend purchasing from a good, local bike shop. First, the bikes they carry are generally better quality than what you'll find at department or other non-specialist stores (better frame, components, tires, saddle, quality assurance overall). Second, you stand a better chance of having the "fit" of the bike taken into consideration with their recommendation, and "customized" when necessary (e.g. longer or shorter stem). Third, it's much more likely that your bike will be properly built and set up -- both from a mechanical perspective and the rider's "fit". Fourth -- many bike shops include a free tune-up after your first 250 miles or so.

A well-tuned bike that fits you properly will be a joy to ride for many miles and many years. An ill-fitting bike with poorly adjusted gears and brakes, cheap saddle, etc., will gather dust in your garage.

All of the above plays into the decision about purchasing a bike at a "super sale". There can be some really good deals at those types of sales. But do your research in advance -- make sure you have a list of bikes (and their associated frame sizes) that you KNOW are going to be right for you. In such an environment (ie. the tent sale), you won't get the kind of personal attention that you would going to an LBS under normal circumstances.