Everybody and I do mean everybody, including yours truly, lusts after the stuff we couldn't buy when we were in our formative years. But unlike some high-quality products, such as Allen Edmonds shoes, which can be bought used on EBAY for a fraction of their new price and then restored by their original manufacturer to near new condition, used HiFi gear isn't as easy to return to original specifications. Two examples of the exact same model can perform quite differently depending on their condition.
Now, a small percentage of used and vintage HiFi gear will work seemingly forever without any attention or repair. But a larger percentage of vintage HiFi gear is the electronic equivalent of owning a sailboat - it's a hole in the ether you continually throw money into to keep your system afloat.
So, how do you tell the difference? The first criteria should be age. The older a component is, regardless of what sort of component it is, the more likely it will need refurbishment. With dynamic driver speakers as the driver's surround ages its physical characteristics can change, making the speaker's frequency and power response less linear. Fortunately replacement surround kits are available for most drivers, but they do depend on the installer's skill to bring a driver back to factory specs.
Most electronics have capacitors somewhere in the circuit. As capacitors age they can leak and fail. The bigger and older a capacitor is, the more likely this is to happen. Resistors' specifications can also change slightly with age, which if they are controlling critical voltages can wreak havoc with active components. Solder joins can oxidize with age which can cause intermittent issues that are difficult to diagnose. Sometimes older solder joins get heat-sensitive, so they only fail after X amount of usage time. And volume controls can get noisy from so many ways, including dirty or worn contacts.
Older turntables present a unique and even more convoluted set of potential repair problems since they are both mechanical and electronic. Getting accurate speed performance can involve repairs to the motor and the parts connecting the motor to the platter, whether the turntable is a belt, rim, or direct drive design.
After age, condition makes the biggest difference between a good value and a bad one. Among automobile collectors "Original Condition" has become the new "mint." But for audiophiles, the idea of keeping a barn-found Marantz 8B amplifier "original" condition, including dead spiders, rusted connections, and broken tubes, hasn't yet caught on probably because we like our stuff to work.
Audiophiles who gravitate toward vintage gear often have more than one of anything they like. That's because if you need parts for an older component chances are the only way you are going to find those parts is by removing them from another unit. Also the more time anyone spends looking for and acquiring used audio gear, the better they get at it. For some vintage-loving audiophiles the quest becomes even more important than the components themselves. And yes, if carried to an extreme, this can turn into hoarding.
So, to answer my original question, used gear can be an excellent value BUT to get to the point where you can begin scoring great values on a regular basis you have to do your homework, and most likely, pay your dues through some less-than-stellar value purchases. I know I have...