Sorry you had problems... From my experience with KEH, they're pretty dependable and would do their best to make right any problems (limited by availability, since they're dealing with used equipment). I have had dozens of transactions with them and only once returned an item that I wasn't happy with... not really a problem (more my mistake than theirs) and they promptly refunded my $ money, no questions asked.

Not sure why you'd send a 5D Mark II to CPS just to install a focusing screen. It's a widely available, $35 item to buy one and very simple to install yourself (And isn't this sort of like having to put new tires or have a brake job done on a used car? With a used car I'd always at least change the oil and install new air & oil filters, maybe do some other preventative maintenance). In fact, I replaced the original Eg-A screen in my 5DII with the Eg-D "grid" when I bought the camera new, because I prefer that type (which Adorama, B&H and others have those in stock as well as other optional screens.. The original 6D also uses the Eg-series screens).

Maybe you just want CPS to go through the camera. I'd check the shutter count first, and if were low might not bother, would just look it over myself, update the firmware if needed, and maybe clean the sensor... the run the camera through it's paces... and only send it off for service if it really appeared to need it for things I don't or can't do myself such as light seal leaks, sluggish shutter, meter calibration.