There IS a certain pragmatic logic to it, though.
One is that since most of the ultralight gear that interests us isn't available in retail shops. So how do you go and try out a Hexamid without buying one? The same is true for TarpTent tents -- they have a great reputation, but the line is pretty varied, and it can be pretty tough to decide which one is going to be ideal for you without trying one out... so you make an educated guess, and try it out.
If it doesn't work out to be quite what you wanted, you can sit on it or sell it. If you sell it, you can get most of your money back and use it for something else. Seems like a no-brainer to me.
I imagine that Henry's selling quite a few of his tents, since they have a good reputation and he puts a lot of effort into making sure that they live up to it.
While not the lightest, Henry's tents are among the most reasonably priced, so it's quite logical that they'd be popular in the first place, which means that they'll be popular second hand, also simply by extension of logic.