Norm - the most important thing about bike fit is reach. Sorry if I'm dumbing it down excessively but that means the effective top tube length plus the length of the stem. If you have a top tube in the right range, you can tweak out fit by switching out the stem. So if you know she's a 45-47 and she knows that she has a long or short torso you should be able to find a decent fitting bike on ebay, but it's a hassle. Here's an example: if your wife is short torsoed relative to leg length... you should likely lean toward the 45 and only consider 47s with a shorter top tube length. That means getting a feel for what the avg top tube length and searching out the geometry on each bike.

Also, if she wants to put tri bars on it, I would cheat a little to the short side because the bars make a difference.

Buying a bike at the LBS allows her to try a bunch of bikes out, find the right fit and when you hopefully buy a bike, you'll get great service.