I would venture to say he is being sold as "gentle for beginner riders".
He is gentle because your eyes are correct, he is thin --- very thin.
He has a contusion on his left hip bone where another horse most likely kicked him. That contusion could be nothing in terms of serious damage or, if he was kicked just right, he could possible have a fractured bone in the hip.
My Arab has carried a contusion identical to that for 15 years without issue, from getting kicked but it is still something to ask a vet about.
I would not buy this horse without paying for a PPE (pre-purchase exam).
But, before spending money on a PPE, do you have any experienced horsemen in your area that could go with you to examine the horse and that do NOT know the Seller or have done business with them?
Lastly, I am sorry but, with you being a first time horse owner I would pass on this horse. There are a lot of well cared for and beginner friendly horses out there, without an inexperienced person taking on what appears to be a lot of work and, for all you know, heart break in the end
What are they asking for the horse anyway?