I haven't followed the Minis after I bought it. Do they all hold their value or is this one special?
In general, all Minis do hold their value quite well, as they are (a) solidly built and (b) are the least-expensive way to gain access to OS X, and are therefore highly sought after.
But yes, the 2012 Mini is in fact special, because Apple has only updated the Mini line once since then (in 2014), and that update had several downsides -- it eliminated the quad-core CPU option, it made the internal drives much harder to access (going so far as using security screws to hold them in place), and it literally soldered the RAM down onto the motherboard, so that it can never be upgraded. The only significant improvement in the 2014 Minis is in the graphics capability, and even that improvement is fairly minor.
So yeah, many folks actually see the 2012 Mini as
superior to the 2014 Mini, and thus the market in 2012 Minis is still quite brisk today.
Also, would you sell all of this for a 5K iMac?
If you
really need an all-in-one computer, I suppose.
I'm not a fan of the iMacs myself, as they are just awfully expensive, and if I'm going to buy a big, expensive monitor, I'm going to want to share it with all my computing devices, not have it attached to just one of them...