craigslist is by category so I'd say if you're looking on planes on craigslist, its more likely you could afford it and be interested than a random doc.
but that is a bad comparison. trying to sell a 500k plane by a paper ad is foolish. I'm sure there are common places where planes are sold. I doubt you'd buy 2k shoes from a fellow doc on a paper ad, I'm sure there are shoe marketplaces where its much more common to sell them.
FWIW I have never once heard a pilot say, wow did you see that plane listed on CL? Looks like a great deal and in awesome shape. There are too many factors that come into play when deciding which plane to buy (from a maintenance and safety standpoint) that I doubt any real airplane purchasers would even bother to look. Not saying there are no planes ever listed on Craigslist, but I am saying I doubt they are the 500k type planes.
Also, just for general information (in case anyone cares) there are 4 different engines in an airplane and each is more expensive then the last.
1. Piston Engine (just like a car engine basically). Least expensive to buy and maintain, but limitations include, often have less power, lose power as you go higher limiting your service ceiling to about 13,000ft, and are the second to last of the 4 on reliability.
2. Turbocharged Piston Engine: more expensive to buy and maintain, but main advantage is they maintain their power at altitude allowing you to climb up to about 25,000 ft. Since they are piston engines just with a turbo they essentially have more things to break with not other features to increase reliability, making them the least reliable of the 4.
3. Turboprop (Turbine Engine with a propellor): Is the second most expensive to buy and maintain, but great feature include high horsepower meaning the plane goes significantly faster, can clim at a very high rate, and have high service ceilings (typically around 30,000ft or so), they are extremely reliable.
4. Jet (jet engine/jet fan engine the type of engine when you think of an actual jet plane): most expensive to buy and maintain, similar features to turbo prop but increases in efficiency of design mean quieter operation (typically), higher service ceiling (sometimes up to 50,000ft, or more of military), faster speeds, reliability is on par with turboprop, both very reliable.
$500,000 can get you a lot of different planes that will be better/or worse depending on your intended use. You will see planes flying from the 70's and 80's and still be very expensive relatively because the airframe has a very long life span and unlike a car, replacing the engines is part of the maintenance of plane. Typically every 2,000 hours as just general rule, and by updating the avionics package every so often they can be just as safe as any of their brand new counter parts.
While many exceptions exist, if I had to say just general price brackets for the engine classes it would likely fall into this.
1. Piston Engine 50k-250k
2. Turbocharged Piston: 100-600k
3. Turboprop: 600k-1.5MM
4. Jet: 1.1MM+ (most expensive non commercial style plane is the Gulf Stream G650 which is something like 80MM new if I remember?)
All of those prices largely reflect the used airplane market. New planes would all likely be double the prices I listed.
Now you know.