I bought a MacPro 3,1 (2008) 2.8 GHz Eight-Core from MacOfAllTrades in October of 2011.
The computer, though used, arrived in fantastic condition. The exterior of the case was flawless (truly like new). The insides of the machine showed use, but was in very good shape and (mostly) dusted. For shipping, the computer was packed in a box with custom sprayed-in foam, so it didn't move at all during transit and arrived in great shape. Parts of the outer box had even undergone some shipping trauma/damage, but the tower was still in great shape as it was cocooned inside many inches of the hard foam shell. Definitely top-notch shipping (aside from an actual original Apple shipment, of course).
Mine arrived with 10.5 Leopard pre-installed (no OS discs). I never even booted it- I immediately cloned the drive (using a MacBook Pro) as it arrived (so I could always restore to its "original" state), and jumped right into new installation of 10.6 Snow Leopard instead. I have never looked back, as 10.6 has worked just fine. I still have the 10.5 clone if I ever need it.
My Mac Pro came without keyboard or mouse- I picked those up at my local Apple Store. The first thing I did with the machine (other than install 10.6) was test all the hardware- every port, every bay, every internal and external connection. I wanted to make sure that everything was working properly, as the warranty was only 90 days. It all checked out. I did experience a few scary random/sudden reboots (no warning, no kernel panics, nothing) early on, but I later traced this to some RAM that wasn't completely seated in the motherboard trays. Once I fixed that (just re-inserting the RAM securely) the problem has never reoccured.
Though my machine was upgraded (larger hard drive, upgraded GPU, and extra memory), the upgrades are not all Apple OEM parts. The pairs of RAM sticks were some cheaper/generic brand, for example. None of the upgrades, though, have failed to perform properly.
In my case, the price was very good (better than the Ebay and Craigslist prices available at the time, and much better than PowerMax.com). My machine has worked just fine ever since I received it, so I wouldn't hesitate to use MacOfAllTrades again if they had what I was looking for at a really good price.
Let me know if you have any other specific questions about my experience. I've only used the company once, but I was quite satisfied. Definitely a legit and professional operation- nothing shady or nefarious going on.