I would think its a scam - but if it is too good a deal to pass up I would proceed with extreme caution and use a temporary e-mail address made on Google or Hotmail, or better yet some Free Russian or Chinese e-mail provider that would be damn near impossible to trace.
I would definitely not purchase it unless I had the ability to inspect the goods in person before handing over any money. I would also demand the place be a public venue. I am sure you can find an outlet at a public library, for example, where you can plug it in and give it a go.
I would not bring the money to someone's shady "house" (it could be a setup for you to get jumped since they'd know you have the cash on you).
Also, once paid, I would keep my eye on the unit and note the serial number so that it is not switched last minute as what happened to someone on another thread with a mac mini.
If everything was working great, I got to inspect it, everything is up to spec, then I would buy it.
I would not ask where the he got it from or why is he selling that cheap. I wouldn't want to know. The less you know the better.
When I'd take it home I would immediately reformat the hard drive (before connecting it to any sort of network or internet) to get rid of any potential snooping software or location tracking software (like lo jack), put on my own things, and never bring it into Apple service, ever, no matter what.
I would not buy it under any other circumstances, I don't care what excuses there would be, I would never send money to have it shipped, do a deal in person without having a chance to inspect the unit and verify it is working, etc. Anything other than full transparency is probably a setup for you to get scammed.
If there is any way the method of sale being proposed by the seller could result in them scamming you then chances are at that price it probably WILL be used for that purpose. Therefore, the entire deal should be on your terms. If the seller isn't willing to comply with reasonable safety steps, then stay away.
Anytime I sell on craigslist, ebay, gumtree, etc. I understand that others may not trust me and I always accommodate reasonable safety precautions, like meeting in public places, allowing the person to inspect the goods in person, I even went to the Apple store once with someone who was buying my old MacBook Air and allowed the person to verify the status of my Apple Care with the staff there before going through with the transaction.
In return, I ask that the buyers pay in cash (the easiest way to ensure that payment will not be overturned, turn out to be fraud, etc.).
