Buying and selling cars is an easy business. This is because most people do not want to hassle with selling a car much. They will not even clean it up, as they just want to get rid of the darn thing and will sell for a low price. All you have to do is make them a reasonable cash offer, and off you go with a vehicle that you can clean up and sell the very next weekend for a nice profit.
In my mid-twenties I worked with a guy who started buying and selling used cars on the side. We got to be pretty good friends. His name was Kelly, and we both worked for a company that wholesale-distributed auto accessories. There were one or two other guys in the company that were doing the same thing. But nobody did it as well as Kelly, he got the whole process down to a science. It was very rare when he held on to a car longer than two weeks, and most were sold the very first weekend he had them for sale.
It was not very long before Kelly quit the job and just bought and sold cars full time for a living. It's what he loved to do. His landlord owned a dirt lot on a street with pretty good traffic and let him put his vehicles out there with a "for sale" sign on them. I always figured that's why he was so successful.
Years later Kelly confided in me that the dirt lot with the sign severely ever sold a car; he pretty much did it all with Auto Trader ads. I asked him what his secret was, and he broke out a small worn-out blue notebook scribbled with pages of notes. In the front was a checklist that every car he bought and sold had to pass. He jokingly referred to it as his "Kelly blue book of car value," a play on words on the famous Kelley Blue Book (the industry standard for used car values).
The bottom line was, he only went after certain high-demand vehicles and they had to meet certain criteria (as well having being able to buy them under market value). His main focus was on 10-year old 4-door Honda Accords and Japanese mini-trucks. They had to run well, have current registration tags, have a straight body, and good tires. The best prospects were dirty and had stained carpet and / or ripped front seats.
Cars that met his criteria could be cleaned up easily and have new seat covers thrown on, plus the carpet dyed, to spruce up real nice. Because the sellers did not envision how nice the car would clean it he could buy them cheaply; and because they did clean up so well he could sell them quickly. That was the trick.
Follow Kelly's example and you can have a nice car buying and selling business of your own.