Pretty much the same procedures as with buying any used computer:
1. Make sure the seller has the receipt, preferably original (if you want to be even more careful, ask to see the seller's drivers license to make sure that he is the same guy as the buyer of the machine; assuming there is a name on the receipt)
2. Check the warranty coverage using the machine's serial number to make sure it is still covered (it should be but there can always be exceptions). It's also a good way to check that the seller is legit and telling the truth.
3. Check the exterior for any dents, scratches etc. If there are something that the seller did not mention, then renegotiate the price.
4. Boot it up and make sure it works (no need for thorough inspection, just make sure it boots fine and etc)..
There are couple of points that I came up with.