Tri bike...getting a used one?

Not reliable at all, but better than just a guess. It does take in consideration body measurements (although it is lacking discrepancies in for example leg length...), but only some flexibility and core strength (not enough data to really be useful), but it's sorely lacking some extremely important data such as goals/aspirations, distances riding/racing, type of terrain, experience as a rider, current/past injuries/issues, etc.

You're buying a tri bike to be faster (I assume... you could of course buy it to look cool, then just skip all I'm writing...), and since the rider accounts for the vast majority of the drag, get a bike fit first, then buy the bike that fits you the best. The benefit of a tri bike is really the position it allows the rider to be placed in, while still be able to generate power and be comfortable. This is why an experienced bike fitter can make such a big difference... The compromise of aerodynamics, power (efficiency) and comfort, all with distances, terrain, injuries, etc. take into account to make the best possible solution for YOU. The aerodynamics of the frame is only second to all that...