I've been to a fair few of these now and the first time is always a bit overwhelming. The catalogue descriptions are usually vague or wrong, the lots are bundled together in ways that make no sense, and there's always a few blokes who clearly do this professionally and know exactly what everything's worth. Don't let that put you off though, because there's still plenty to be had if you know what you're looking at.
The stuff I always go for first is combi ovens, particularly Rational. They cost an absolute fortune new, anywhere from five to fifteen grand depending on the size, and they hold their value better than almost anything else in a commercial kitchen. Even ones that aren't working have decent value because people buy them for parts. I picked up a Rational SCC 10-grid from a hotel in the Lakes last year for about a third of what they go for new, and the only thing wrong with it was a dodgy door gasket. Twenty quid part, ten minutes to fit. Sorted.
Stainless steel prep tables are boring but they sell all day long. You can't really break them. A clean and maybe a new set of feet and they look like new. Easy to shift, easy money. Same goes for stainless shelving.
Glasswashers and undercounter dishwashers are a good shout too, especially anything by Classeq, Winterhalter or Hobart. Pubs get through these constantly and a decent used one will always find a buyer. I'll do a proper write-up on dishwashers separately because there's a lot to say about what goes wrong with them and what's worth fixing.
Vacuum packers are another one to look out for. The cheap domestic ones are worthless but a proper commercial chamber vac from Henkelman or similar will hold its value really well. They're not cheap even secondhand, but there's always demand.
Now, the stuff I'd avoid. Built-in extraction hoods and canopy systems are basically immovable unless you're fitting out your own kitchen and it happens to be the right size. Not worth the hassle. Three-phase equipment is a smaller market than you'd think, because most smaller premises and definitely domestic properties don't have a three-phase supply. I'm not saying don't buy it, but be aware you might sit on it for a while. Ice machines look tempting but they're maintenance nightmares. Limescale, blocked water lines, refrigerant leaks. Unless you know your way around refrigeration, I'd leave them alone. And cheap no-name fridges from the catering catalogues aren't worth bothering with secondhand because new ones are only a couple of hundred quid and nobody wants a used one.
One thing worth doing is turning up early and having a proper look at everything before bidding starts. Check power cables and plugs. If someone's hacked the plug off an oven, that tells you something about how the rest of the place was maintained. Take photos of model numbers and serial numbers so you can look things up before you commit. And bring a van, because most clearance sales want everything gone the same day or the day after.