It easy to forget that there is a large used market for computer electronics. Many people see PCs as relatively fragile pieces of hardware, and assume that this means they should take the precaution of buying only brand-new components. In fact, individual pieces of hardware are fairly reliable. Processors seem to last forever, provided they are not subjected to over-clocking. Video cards can also last a decade or more when properly treated. RAM is very durable - for the most part, you only need to be concerned if the hardware has been snapped in half - again over zealous over-clocking aside.
There is also a large market of returned and "Open Box" items available at retailers. The fact that items have been returned does not always mean they are defective. Usually, someone less careful with their research than you finds that their purchase just does not work with another component; or a person attempting an upgrade discovers that their new hardware does not fit in their current PC.
The obvious advantage to buying new is that there is little risk in your purchase. Your purchase is usually covered by a manufacturer warranty, and most retailers will take back defective products for a full refund for at least thirty days (check this, many discount retailers will point you straight to the manufacturer's warranty). Should anything go wrong, you can simply get a refund and walk away. The disadvantage is also obvious - buying new means paying a higher price.
Returned or "Open Box" items tend to occupy a middle ground between new and used products, but their overall value depends on the backing of the retailer selling them. Some retailers will only take back an Open Box item if it is brought back within a week, while others will take the product back for thirty days or more, so make sure you know the return policy of the retailer you're buying from. Most returned items are still covered by the manufacturer warranty, but again, make sure.
Used items typically cannot be returned because they're purchased from individuals or from small used-component dealers on websites like Ebay or Amazon. Because products usually can't be returned, you are taking a certain amount of risk. This risk can be mitigated, however, by only buying from sources with a good reputation. Purchasing from an Ebay seller than often gets poor reviews is not a good idea, but buying from an Ebay seller who has received hundreds of positive reviews is nearly as good as buying from a regular retailer. Purchasing from an individual usually means you don't have an easy way of judging their reputation, but if you're making the purchase from the For Sale section of a popular tech website, you can look at their seller's earlier transactions to see if the buyers ever had any reason to complain. Used items are sometimes covered by the manufacturer warranty depending on: the warranty's terms; how old the item is; and any modifications the previous user might have tried to perform.
Refurbished items fall in between used and open box. They have been used for some time and sold back, not used briefly (or not at all) and returned. But they go back to the factory (or a third party) to be cleaned up and tested. You can value these, indeed any of new, open box, used, or refurbished, products, based on the warranty available.