I get a lot of questions from people looking to buy pianos. Everyone wants to get a high-quality instrument at a good price. However, just as with cars, what that actually means is different for everybody, and so there is no quick answer.
Different manufacturers are associated with different attributes. Steinway and Bosendorfer are perhaps the Mercedes and Porsche of pianos, while Yamaha, Baldwin, and Kawai can be compared to Volkswagen, Volvo, & Nissan. Maybe Wurlitzer, Young Chang, and Kimball are like Chevy, Hyundai, or Saturn. Further, each manufacturer has different models at different price points, and within each model, there are various trim levels. It becomes a matter of personal preference whether to get a fully loaded Camry or a no-frills Lexus.
Once you get into the used market, the similarity between piano shopping and car shopping continues to hold up. While you take a huge hit on depreciation if you buy new, there is a strong used market in both cars and pianos because of their long and predictable lifespan. Of course, pianos do not have odometers, but there are ways to check for wear and get a sense of the degree of use the instrument has been subjected to.
If I were going to buy a used piano (and I do it a couple of times a year), I would look on Craigslist first. Just as with car ads, you will find a mix of private sellers, small-time dealers, and the occasional big-time dealer (luckily, still far fewer scammers than on the auto listings). As you might predict, you will get better prices but less convenience and information from private sellers - and dealers will be more firm on price but hopefully you will be able to check out a few pianos in one trip, saving time and allowing side-by-side comparisons. Just stay away from anything that says "antique" - tall old uprights will have a powerful sound, but will not pass emissions testing, so to speak.
Once you have explored Craigslist a bit and perhaps checked out a few pianos, you will probably have an idea of what you want to spend. However, just as with cars, you can occasionally find a good "moving special" where you can upgrade to a better piano for a below-market price. And obviously everything is negotiable, especially in the current market.
Even if you are not an experienced pianist, you can often tell if a piano is a good deal with a little advance research. What's more, you can even hire someone to check the piano out for you - akin to hiring a mechanic to come out and check to see whether a car is a lemon. In any case, don't be afraid to kick the tires and try out the instrument a bit yourself - gut instinct is valid, so don't be intimidated if you're not a musician.