Unfortunately, some people lie. I would find a vet that will do a pre-purchase exam for you. While that may cost you $200 or more, depending on where you are, you are making a big commitment and that is really a small price to pay to know what you are getting into. It may be simply neglect and he will be fine, but it could be more and end up costing you both financially and emotionally. I bought a horse many, many years ago that was in very sad shape, after determining that it was just from neglect and he was sound otherwise. He needed time, feeding and care and he was ultimately wonderful. But during recovery he colic-ed and also had a bout of large scary hives.
As hard as it is, making an emotional decision about an animal like a horse can have terrible consequences. That looks like more than 3 weeks of neglect to me, but the saddle in the first picture is hiding a lot. People lie about age also, if he's older, has not had teeth floated and is having a hard time feeding, that could also contribute. I have called animal control on horses that look like this before, did not feel bad about it at all. The animal deserves better.
As hard as it is, making an emotional decision about an animal like a horse can have terrible consequences. That looks like more than 3 weeks of neglect to me, but the saddle in the first picture is hiding a lot. People lie about age also, if he's older, has not had teeth floated and is having a hard time feeding, that could also contribute. I have called animal control on horses that look like this before, did not feel bad about it at all. The animal deserves better.