Interesting idea. However, it's not difficult to figure out which used bike is going to fit, even if you're buying online. There are plenty of online fit calculators available now, and anyone with a little time to spend and a friend with a tape measure can tell which bike should work.

I would like to believe that a bike shop employee wouldn't put someone on an ill-fitting bike just because it was in stock, but I know I would be wrong. I remember being in a bike shop in Gainesville,FL about 15 years ago, and seeing a titanium Mongoose road bike on the floor. I straddled it and it was two sizes too large.Bike was a 56, and I usually ride a 52. Salesman(kid) was on me with the hard sell right away. If I didn't know it was too big I could've been sold an ill-fitting expensive bicycle.

One can always pay the LBS for a fit session, and use those measurements while looking for a used bike or an online special.

Having a friend who's into bikes is the best option.