I agree entirely that is is nice to hear a good sound sample.
Now come the buts ...
Not every luthier is a superb player - probably relatively few in fact. A decent recording set-up takes some cash investment and definitely some time investment to get it all working properly. If you ask some professional to play for the recording then, then they should be getting something for their time. If you assume when buying direct that something like 30%-50% of the price is for materials and overheads then any additional cost (both in time and money) knocks a chunk out of the makers pre-tax income.
Then there is choice of music - some choice of music will put off some listeners completely. I have heard some sales recordings that I could not listen to to beyond a few barres, because the choice was way outside my taste range, despite the skill of the player. If the quality of the recording is poor then it is counter productive. Then there is the thought that people who hear a recording deciding (and posting all over the net) that a particular luthier's guitar have poor trebles whereas in fact the truth is that the person has a computer with crappy speakers and is used to recordings that have been totally manipulated.
I saw a post a one point where someone was considering getting a luthier to build a guitar for him and was asking for advice about wood, body shape etc ... there was a link to a recording (not from a seller or maker) which he liked and the question was what design would give that sound. The real answer (slight exaggeration
) was that a plank of wood with nails and six strings could sound like the recording in question - there was so much reverb added that you really couldn't say anything about the sound characteristics of the guitar. Moral of the story, there are dangers as well as costs attached to recordings.