Dot com auctions are also often good places to get non-geek things like office furniture and supplies. More often than not, your competitors will be other geeks trying to score good deals on mildly used equipment (and usually failing; people will drive the bids up wildly). But dot coms were also known for other examples of conspicuous consumption, so you can walk out with a ream of copy paper for $10, or a few $100 secretarial chairs for $40, or even a refrigerator or microwave for under $200.
If you're going to an auction held by just one company, beware. The smaller companies, especially in smaller areas (like New York, where space has always been at a premium) will sell computers individually, and will rarely consolidate them into lots.
And finally, What You See Might Not Be What You Get. Keep an eye on what you buy. In certain crowded auctions, I've seen people go behind the auctioneer's back and tinker with the equipment after someone else paid for it.
Update 12 April: the events in the above sentence happened to my company: we bought a computer and left it overnight; in the morning, the RAM was missing.
The moral of the story is: Be careful.