I did well a couple times - got a practically new 5' rotary cutter (Land Pride) for $800 - I'd been watching "locally" (within an hour's drive) for years and only seen seriously beat up mowers. The cutter I got at the auction was missing a piece in the top linkage, which was about $30 to find online. Add ~15% to that $800 for taxes and auction fees, if I recall I'm in it about $1100 total.
Also got a 2 meter tiller (a bit over 2m outside measurements), cat I/II. Super heavy duty, completely unused though the paint says it was outside a long time. The tines were untouched. Immediately lost a tine first time I used it, but found it in the dirt; proceeded to pull every tine off and loctite them all. It's a monster and my 30hp PTO requires that I go really really slow in virgin ground, but I'm not tilling acreage with this. The slip clutch lasted three seasons and I had to scramble to find a bushing and the friction plates. That tiller was $1000 before fees; considering equivalent new prices I think I stole it.
Also bought a pallet of redwood 2x4's and made a bunch of large raised-bed planter boxes with them.
I did buy a couple air compressors that were absolutely garbage; pure loss. Considering the bargains I got before, though, I'm ok with that, caveat emptor.
For a while I was hoping to find a dump trailer or flatbed there but they're obviously getting top dollar for those; rather than go near full price at auction I'd rather get a top name with at least a pretense of a warranty and spec it out how I want it.
Auction place is inconvenient for me - about 1h10m away from home, so unless it looks absolutely amazing I'm unlikely to bother previewing which means I have to get it even cheaper... lately I can't be bothered to drive an hour for much of anything.