I'm about to purchase half a dozen slightly older servers. I see a lot of pretty good specifications quite cheap on ebay. So it won't be the latest CPU but it would still be good enough. The machines will all run Linux (Debian) and will be 64bit.
What brands would you recommend?
What gotchas should I be looking for?
Is any particular features in those slightly mature machines a must have?
When I buy a used previous generation server, I look for a vendor that sells used systems in volume and will provide a warranty. I personally have had good success with used Dell servers from Aventis Systems, but there are other reputable vendors out there.
If you want to buy on eBay or other non-reputable sources, consider the need to buy multiple items of the same model and maintain spares and/or parts inventory.
You have not specified what you are planning to do with half a dozen servers, so it is impossible to recommend a feature set. Generally speaking, though, you want failure-prone components like drives and power supplies to have redundancy.
Miles is right. You should be looking for a reputable vendor that will provide a warranty. They are not difficult to find and it is a nice balance between servers coming in an unknown condition and a new product.
Buying on eBay should be a last resort for an enterprise environment. If I were buying for my home or something sure... but only in that case really.
Good luck.