Why Purchase used tubes?

Joe's Tube Lore is an excellent guide, though I have found quite a few tubes to vary from his descriptions, depending upon the component used. In two different amps, for example, the Mullard 2493 (6922 equivalent) sounded much more balanced and tonally pure than either the Amperex Pinched Waist or the Telefunken CCa. A friend and I also heard the Pinched Waist in his DAC, and they sounded more etched than his stock tubes. Go figure. I have yet to find an Amperex (save for a 6X4 rectifier) that didn't sound too bass heavy...could be the room, who knows?

The guides are useful, for sure, but your own ears will be the final judge.

Now, the Siemens 1960s (not 1970s) CCa's...yeah, those are sweeeeeet! Definitely get the MBG, however, in case they are microphonic.

Warren:
Like you, I have been, comparatively, a late arriver to the 'tube party'. However, once there, I chose to get involved. It is good that you have a mentor that you trust...it will save some headaches. With that said, there is a wealth of info on various tubes if one does his homework. With all of that said, IMNSHO, as the old saying goes "in your system, in your environment......". What one person loves and raves about may just as well NOT be so good in your system. Experience, and adequate tube rolling and listening is the REAL teacher.

In my experience, I agree with Matty that the wear curve is NOT linear. I also strongly agree with Kurt_Tank about being sensitive to the 'bang for the buck' realities.

As regards your first question, about buying truly NOS, in my experience, some of the REALLY desireable tubes are getting the point of being SO picked over that sourcing TRULY 'New Old Stock' tubes is becoming questionable at best. Does anyone REALLY know the history of the tubes that are landing in their hands? It would seem that the best that anyone can do is test and quantify.