If you can get the right size and the price is right for the condition the bike is in, then buying used can be the way to go. As you indicated, there are some freebies you won't get if you buy from a shop, but you could still easily come out ahead if you buy used.

As for "free fittings," in my experience, few shops offer a free comprehensive fitting. They more or less just eyeball it, which you can do fairly easily on your own. If you want a more comprehensive fitting, whether it be on a new or used bike, you have to pay extra for it. Check around at your local shops to see if any of them offer a fitting service. I bought a new roadbike a few years ago and paid to have a fitter at another shop fine tune my fit on it.

Obviously, a used bike won't be warrantied. I'm assuming the bikes you have in mind are mostly aluminum with some carbon parts. If so, I wouldn't let the lack of warranty be a deal breaker. As for financing, more than likely a private seller is going to want cash. In these economic times, I wouldn't necessarily recommend financing with an LBS though, but then again, I'm a big proponent of buying only what I have cash for. If your money is really that tight, you might be better off buying used if you can find the right bike once you have the pennies saved.