In terms of buying aeroplanes, there are various websites that list available aircraft. Although Autotrader has not yet entered the market and Exchange & Mart are viewed with some suspicion in aviation circles.
The one of the aviation equivalents most popular in the UK is
AFORS (Aircraft For Sale). This covers Group A as well as Microlights. The usual caveats about buying second hand and advertising apply. It will give you a general idea of asking prices at the moment; whether those will still apply when you come to actually wanting to buy is an open question.
The overall aircraft market is not quite as straightforward as the motor trade. Some Group A aircraft are relatively cheap to buy, in part because they are expensive to run. Some aircraft have components that must be changed after a certain number of flying hours or years in service. Predictably aircraft parts are specialised and expensive. The turn over of aircraft is far slower than cars; much fewer new ones are bought each year. In general terms aircraft are better maintained than the equivalent car, but the technology is lagging (when did you last encounter a car with magneto ignition?)
Enjoy the learning process and keep asking the questions.