I agree that BikesDirect is a worth considering if you're not sure what to look for in a used bike. You still need to know what size to get and you won't get a chance to ride before you buy. Also, unless you intend to get one of their higher end road bikes, I'd set some money aside for new tires. The tires on their cheap road bikes have no real flat protection.

There are good deals to be found on craigslist if you're patient and no what to look for, - especially this time of year if you live in a colder climate. What may work better than looking through all the postings is to create your own "wanted" post telling people what it is you're after. You can specify a size range, no steel wheels, or whatever criteria you feel is important.