While this story really sucks and upsets me - I think you have to chalk this up to a good lesson learned.
In all reality it is only $300 and the amount of time you've already put into "investigating" this and your safety - has to be worth more than the $300 you lost.
Well that assumes that the spare time the OP (or any of us for that matter) can be put to a higher value use.
Unless the OP is foregoing generating money in favor of investigating this, then it could be worth the while. $300 is quite a bit to some people as well, so maybe it is worth chasing. I assume if someone was looking for bargains on craigslist they didn't have the money to buy a new unit or it would have presented a hardship.
The lesson here is though, consider your options carefully. A lot of people, and who can blame them, try to get the most bang for their buck and are especially tempted by the amazing deals that come along (which usually border on sounding too good to be true). In all aspects of life though, the cheapest option isn't always the best option, in fact it rarely is. Usually its a better idea to pay the extra few bucks and be guaranteed of your choice.
In the end things usually balance out financially, and the cheaper option usually costs more in time and hassle.
In this case, attempting to save $300 cost the OP a Mac Mini. The next $300 worth of bargains the OP will get in his life, assuming they all work out, will only balance out the loss incurred here in this one incident.
I too was really cheap once, thinking I'd save a few bucks, so I got a taxi cab to help me move my stuff instead of a proper moving service (I didn't have a lot of stuff, just one car load worth). I saved maybe $20 over hiring a private moving service for the hour, but one of my bags must've fallen under the seat of the Taxi and I am missing two miniDP to VGA/DVI cables, a few USB cables, and some blank CDs.
The cost to replace the items is far greater than the $20 I saved by being cheap and getting a cab instead of a moving service. I have no doubt had I hired a moving service/van none of my bags would go missing cause they'd all be in one place.
The same goes for buying used cars versus new cars. You save money now, you pay extra in repair costs and hassles later -- unless you get very, very lucky, but that's rare.