My go-to for awhile has been NewEgg's Refurbished section. Yes, NewEgg is still around! They've steadily had many great refurbs for many years, including mini-PCs.

I like the mini's, but I find they overheat easily, especially if neglected. Having cats/dogs is also a contributing factor. The mini's have one or two small fans and a very compact design. Some dust/fur and enough time and it will overheat and swell the mobo capacitors. I found that buying a new refurb is cheaper than my time trying to soldier replacement caps, which I've yet to get to actually work.

Like the article mentions, though, it is a minefield and you really need to know how to gauge what you are looking at to buy a good deal. There are a ton of very old machines, which aren't the greatest, like 2nd/3rd generation i5's and such. You really need a little grasp on the confusing weave of i3/i5/i7, their generations, processing speed GHz and core counts. Then comes the balancing act of those benefits, versus the extras. Does it come with a good amount of ram? Can it be upgraded to a descent amount of ram? Is it DDR3 or better ram, rather than something ancient. Do you need a PCI-Express slot and a good power supply to throw a video card in? Does it support USB-3? Does it come with Wifi or other extras you need? etc...