The externals wont tell you much, its the internals that matter most. best bet is to get him to charge up the battery ( or borrow a fully charged battery ) and go check the gun out yourself. The externals are easy to inspect ( mostly cosmetic and dont matter much really) almost any external part short of a few screws can be replaced by ordering it in-canada.
Internals would be tricky... best try the gun out and see if the gun shoots right, you shouldent try taking it apart yourself to give it a look over.
-Make sure the hop-up works and can be adjusted
-Shake the gun around a bit ( does it sound like its got a loose part in it? )
-When you fire does it fire properly? ( safe/semi/auto are all fully functional )
-Does it fire fast/slow or is there a big delay from pulling the trigger to it firing?
Overall if you are happy with it then purchase it, if not then think about it... you can offer him a lower price or look around for another gun for sale.
In the end it is Buyer Beware.