Viento,
Regardless of whether the seller has seen this spread, I highly recommend that you repair this crack if you're going to get the guitar. The crack has clear edges to it and is not level, so it certainly needs cleating (at a minimum). But, before you do that...
This could be an example of a humidification problem, but it's hard to tell. A luthier needs to have a close look at the structural damage.
If you think this guitar has dried out from low humidity, I recommend immediately taking off the strings and putting it in a hard case with dampits and damp sponges in a ziploc back. Do this for about two weeks, checking the sponges/dampit every other day. Only once it's properly humidified will you know whether the crack is going to heal itself, in which case it should be a minor cleating repair. Hopefully, the guitar will absorb the humidity and the crack will level out and seal up. But intervention is needed one way or the other.
That should be a bargaining chip for you in your discussions with the seller.