I have no idea who the seller is and mean nothing personal against him... He might well be completely honest so please don't treat my comments as accusations.
But when it comes to buying and selling from strangers my rule is to take anything that a seller says with a grain of salt unless it can be verified.
So all you know for sure is that this is a 1D Mk3 that by all accounts looks in excellent condition (assuming you've handled the body yourself and can verify this), with a serial number of 53xxxx. Although you don't need to tell us the exact serial number, I hope that you know it yourself.
The first thing you should do is look at the following document which is Canon's list of serial numbers which do not need a sub-mirror fix. My own body is on the list, and I can say that I've been more than happy with my AF so if your serial number is on the list then that might give you a bit of confidence (but again, there is no guarantee).
Secondly, are you sure the seller bought the camera brand new? Jan 2008 seems quite late for a new 53xxxx serial number body (I received my body in October 2007 and it has a serial number of 5448xx). Back then demand was higher than supply, so it's not like these cameras would have been gathering dust in stores waiting to be sold. Maybe he's only put 50 clicks on the body, but if he bought it second hand then who knows how many it's done.
Thirdly, here are the links to a couple of Canon announcements regarding the submirror fix. They might provide you with a bit more info (take note of the dates too in relation to when the seller claims to have bought the camera).
And fourthly, as has already been suggested, call Canon and check it out with them. Just because the seller can show you a receipt for a 1D MkIII from Jan 2008 doesn't necessarily mean he bought that 1D MkIII unless the receipt also has the serial number on it. If you're lucky then you might be able to take the body into a Canon service centre where they can confirm the shutter count and tell you more about it's history and whether it's had the submirror fix, etc.
I realise that I'm not in a position to know anything about the camera or the seller apart from the small amount you've told us... but something just doesn't sit right with me at the moment. I'm not saying the seller is scamming you, I'm simply advising you to be cautious.