See How to Buy A Stereo System (without winning the lottery)
It is possible to buy high quality audio/video equipment on Craigslist and save a great deal of money if you know what you're looking for and you're patient. It's also possible to waste a lot of money. One trend I have noticed lately is people offering vintage audio equipment, sometimes for far more money than the demanded when new. Sometimes vintage audio is a good buy, but very often it is not.
Most of the equipment so listed are receivers from Marantz, Kenwood and others built from around 1970-1985. Many speakers are also offered. Most of this is a waste of money. FM Tuners from the period tend to be good. They are analog, which means they don't save your station but tuning to a good FM station is quick and accurate with the meters provided, and because they don't have to synthesize a frequency to tune to. Essentialy, modern digital tuners can be very good but the synthesized frequency competes to a degree with the received signal, and thus make it harder to get high sensitivity with a modern tuner. The amplifiers can be more robust, and more tolerant of low impedance loads. So they can sound pretty darned good. But they are not as a rule, better than a good quality modern home theater receiver.
"Vintage" speakers are often much worse. Speakers are mechanical devices and the materials and methods available to modern designers vastly surpass what older engineers had to work with. While a fifty year old Bozak Concert Grand or JBL Paragon still sounds pretty darned good, it is not the marvel it was in its day. The biggest difference will be noted in the high frequencies. The modern tweeter is simply vastly better than its vintage ancestors in frequency response, dispersion and power handling. In addition, because they are mechanical devices, speakers age. The capacitors in the speaker's crossover may age especially poorly, with clear sonic costs. The suspension of the drivers also age. They need to remain highly flexible to do their job and age is hard on more than the human body. Often drivers need to be re-coned to hold the right sound. This can get to be pricey when applied to multiple drivers. Generally anything built after 2000 is a decent bet but may still need re-coned. Always audition before buying.
Electronics also suffer from capacitor aging. Different types of capacitors age differently, but replacing all capacitors is often required to bring an amplifier or tuner back up to its original performance level. Generally the most expensive products used the best possible parts, and thus they age better. But age catches up with everything.
Second, vintage equipment will have only RCA (or on very high end gear) balanced connections. No optical fibers. No HDMI. HDMI wasn't invented when they built that stuff and the standards keep changing. You'll need to feed it from compatible equipment.
But really good old amps and pre-amps still are really, really good. They continue to have their devotees. If you come across a used Dynaco Stereo 400 you'll quickly find it has it's own online community and people who will happily advise you on restoring it, or making it sound even better. Crown DC 300s, Phase Linear 400s, old McIntosh, Marantz (the 60s for Marantz) separates will make your ears sing. They knew how to build a good amp back in the day, so long as you're not terribly worried about power consumption. To me, this is real vintage equipment.
Still, if you do want great sound and want to save money good, used equipment is often a bargain. Without mentioning any particular brand I'd like to point out places where money can be saved in quantity. First of all, instead of a receiver you often are far better off purchasing separate components. Integrated amplifiers, pre-amps and amplifiers can often be bought at bargain prices. For some reason people do not like to combine and will pay for it. A modern used FM tuner is readily available for under $50. Often a tuner and amp can be purchased for far less than a 'vintage' receiver for the same price. High end separates (amp and pre-amp) can be bought for a quarter of their new price (or less) from very high quality manufacturers if you are patient. I have seen equipment from Outlaw Audio, Adcom, B&K and others very reasonably priced.
When you look for speakers bookshelf speakers often sell for far more than better sounding floor-standing speakers. People like small stuff. True, it is far easier to fit a bookshelf speaker on a shelf in a crowded dorm room. But if you're serious enough to place your speakers properly (generally out from the walls) you'll need to put them on a good set of speaker stands and on stands you've lost no space going with a floor stander and dispensed with the cost and potential problems of a stand. The larger cabinet can be very useful when reproducing bass, so here full size speakers are the bargain.
So if you want great sound on the cheap and can afford to wait a bit, buying a used set of separates and floor standing speakers you can really save a lot of dough and have it sound great. Vintage audio as practiced on Craigslist, is usually an excuse to grab extra money for Grandad's old stuff. No receiver from the seventies is worth a thousand bucks. But good used equipment is worth looking into.