Actually, someone buying a first bike is VERY likely to need a GREAT relationship with a shop. When 500-600 will give you a solid ride, why not get in good with a shop? The free service and the help and advice will be more than worth it. Guardian Bikes

I bought our previous bikes at a garage sale, and a shop. Both were lightly used and cheap. They lasted us for 8 years. But then I don't need a shop, because I have been working on bikes for more years than that.

First bike, get it at a local bike shop. Without question. That is my advice to 99% of people.