When purchasing items for corporate IT, one can use the same resources as you would for yourself. Ebay, in particular, lists just about anything you might need for a variety of IT supplies.
Recently, we discussed the HP Superdome: on Ebay, there are a lot of Superdome parts and products available.
There’s also HP Integrity servers available, as well as parts.
What are the dangers in this? Double-check your service agreement and make sure that you can still service your machines as you expect. Service contracts will often allow you to use some third-party and used parts without losing your contract, but then if anything points towards the third-party part the technician stops and you are on your own.
Used parts might be able to be put under a service contract, but you should check with the vendor first before you buy. This will vary from vendor to vendor and even from product to product, so check first.
Another danger: you must be sure that you know what you need, and that you are getting what you think you are. In buying parts, it helps to have the exact manufacturer’s part number and to make sure that that is the part you are buying: ask the vendor if there is any question at all.
If you are buying a complete system on eBay – make sure that it really is complete. Many (most?) are not: parts may have been removed (though usually these are noted). Parts missing often include memory, hard disk, other drives, even CPUs – parts can often be sold for more separately than together in a system.
Check the reputation of the vendor and the length of time they have been active on eBay; this will help you avoid unscrupulous vendors.
Also, be ready to buy: if you win an auction, you will be called upon to purchase the item. If you can’t afford it, don’t buy it: this should go without saying, but people seem to miss it.
I’ve purchased Sun workstations on eBay myself; there’s a nice collection of Sunfire equipment available. The used market on eBay is tremendous, and you can get some excellent deals if you are careful.
UNIX Administratosphere 
As far as ebay and excellent deals go, something that might be worth trying is searching eBay for listings that have main keywords (maybe brand names) misspelled in the title.
Most searchers will never see these items, so there should be less competition and you should have a better chance of getting a good deal.
You can use a site like ebuyersedge.com (typojoe.com, etc) to search ebay for common misspellings of keywords you enter.
Ebuyersedge also gives you the option of saving your favorite searches, then sending you an email when an item is listed matching one your saved searches, giving you a jump on other potential buyers.
This works best with ‘Buy It Now’s, but is also effective with auctions.
I’ve read that some people actually profit from buying these items and then turning around and reselling them on eBay for a profit. I don’t know about all of that, but it is worth a try to at least find yourself a good deal.