With greatest respect for all viewpoints expressed here (and for the individuals expressing them), I would suggest that a brief sound sample, even a fairly primitive one such as I sometimes make on my cell phone or laptop computer, even though it wouldn't impart a totally accurate representation of the tonal characteristics of the uke, would still likely have the benefit of giving a prospective buyer a pretty good idea as to whether the uke is intonating accurately, which, to me, is one of the most important considerations in making a purchase.
Of course, if any of the strings happens to be slightly out of tune in its "open" position, this could "skew" the overall perception of intonation accuracy once we start playing chords; so when I request a sound sample, I ask the seller to start by striking each string individually in the open position, just to let me hear that the uke is accurately tuned to begin with, and then to play through a song or a few chord progressions. Sure, it's not a perfect science, but it will at least tell me if the uke appears to have serious intonation issues.
I realize that this thread isn't about Got A Ukulele's review methodology per se, but I do like the way that Barry, in addition to commenting on whether a uke is loud vs. softer, will also make observations pertaining to the instrument's "jangle", "shimmer", "sustain", or "bark", or otherwise comment on how its overall sound strikes him (e.g., with regard to the favorablyly-reviewed Famous FS-1, "a little bit thin for my taste"). I find these insights immensely helpful. Plus, he always makes sure that a uke is tuned accurately before he starts playing it. And he acknowledges that "personal preference" always plays a significant role in these matters, and that a uke may not sound exactly the same in your room as it does in his.
Back to the original issue-- is it okay to ask for a sound sample when considering a purchase? I think it's fine, with the understanding that such a sample may be of somewhat limited value-- but it doesn't hurt a thing to request one.
As one of our well-known Forum friends likes to say... "that's my two bobs."