John's suggestions are excellent ways to get an idea of what "street value" actually is on a product. Such things as supply and demand and condition of the product can drastically affect what the value of an item is though. I have seen prices on used gear fluctuate drastically within a relatively short period of time.
Audiogon offers their version of "blue book" pricing. This info is compiled from units advertised on Audiogon, Ebay, etc... They do the work of keeping track and listing / categorizing each component by make and model and you simply pay for their services. This can show you the high prices, low prices, average price, etc... over time. This will give you an idea if something is rising in value, holding steady, falling, etc... If you do a LOT of buying and selling, it is kind of handy. As John mentioned though, you can do some of your own homework by checking various sites and drawing your own conclusions though. This surely is cheaper, but not nearly as convenient. Sean