Aside from CL and ebay, I do not have a used equipment resource. How do you find an equipment auction? When the tool has moving parts, how do you test?
Auctions - family business or farm, preview the listing and if it looks good enough on paper and pictures then you have to go to the site for the sale (if it isn't an online bid). Have your number in your head and don't exceed it once there.
Surplus - state, city, town, federal surplus sales (call or look online).
Let Go, Facebook, craigslist, ebay, a couple of equipment sales websites mentioned on this site periodically
You have to spend some time looking, it doesn't happen overnight usually (never for me but I hear it happens for others that way). As far as inspection goes, start with finish (rusted, missing parts, bent dinged scratched). don't let rust scare you, see if it is surface or deep neglect. Check for pieces being chipped or warped. check the grease fittings, check the oil stick/port/opening. Check the auger, see if it well worn or just scratched up a little. Check the PTO shaft. Spin it by hand while listening. A PHD is a surprisingly simple piece of equipment.
IN the end, you have to make a decision based on what you have seen. I have done this numerous times, and sometimes I haggle with the owner because I didn't get a warm fuzzy when looking at the piece. Sometimes I have driven an hour for nothing but a return trip home. I am not a pro, but I have bought used cars, equipment, furniture through CL, EBAY, auctions, and ad papers/bargain papers, and the old newspaper. I'll drive 2+ hours to save myself a couple hundred dollars, some people won't.