Oh my goodness, he is ADORABLE!!!! I think I fell in love as well!!
HOWEVER... being as old as he is, are you going to be able to keep him till he passes? It will not be easy to re-sell him after you buy him, and besides, I feel that a horse that old should have a forever home. He deserves it.
Now on to the critique.
Slight bit over at the knee, you'll want to vet-check for any major unsoundness there. He has some serious ewe-neck muscling, I would recommend asking if he's a cribber, and if not, you will have your work cut out for you asking him to travel in a frame. He is sway-backed, but not to the point I would be concerned with. He has the cutest little head, and looks like he can MOVE!!
The following is information about senior horses.. I am NOT trying to deter you, I think you are wonderful for wanting to give an older horse a home, it's just information for your consideration.
With a senior horse there are special concerns that you need to think about; at that age arthritis is nearly a given, but can easily be managed by a few methods: Legend injections, herbal remedies (like Wendel's "Easy-Mover", Omega Alpha's "Sinew-X", etc) or not-so-herbal remedies, such as Recovery or Renewal.
Second is diet. Some senior horses keep weight on really well, others struggle with it - ask the owner what he eats, and how often. A hard keeper can run the feed bills through the roof, believe me!
Many older horses can work quite late into life, but some can't handle working (as much or as hard) once they get up there in the years - a vet check can pre-scan for any unsoundness that might effect this, but are you prepared to be able to give him the time off he needs?
The reason I point out the above is that many people get older horses, use them for a few years, then dump them as soon as they're "unusable" which I find outright disgusting... please consider everything!! This pony looks absolutely amazing, and deserves a great home - if you can provide that, then absolutely go for it! I would commend you!!
Best of luck if you decide to get him!! He looks SUPER cute and fun!!
It's a PONY (he's just 13.3). I think he's very very cute and with this economy, his age and his height the price is just right. But that pic with the rider she looks way too big for him. If you are a kid or very small adult that would be a great purchase. But if you are taller and/or on a heavier side I'd stay away. Horse ideally should be carry up to 15-20% of it's own weight (depends on breed) and this one doesn't look like he weight whole a lot himself. 
