Online bike purchasing - new or used - makes great sense if you already know what size you need, and you have already been fitted, so what you're looking for is another bike which you can set up the same as one that already works. That's how I bought 2 of mine, a new Bianchi 928 back in '06, and a ~'92 Battaglin steel frame thatI built up. In each case, I already knew the size of bike I was looking for because I had previously bought one locally and, once I had built up to riding longer distances, gotten a custom fitting.

If you're buying your first road bike? Not so much. No, you don't get the 1 hour custom fitting, but you can keep coming back until it's right, and often the LBS will swap bars, stems, and saddles till it IS right. You MIGHT be able to properly estimate the proper size, if you have a tape measure and a patient friend.

Sure, you can get a poor-fitting bike at an LBS, especially if you're a novice shopping for bargains and go to a shop trying to unload product. It happened to me, with my first road bike. I went to a MUCH better shop for the second one!