Build a trusting customer base. By adjusting to your customers' needs and consistently providing good services or products, you get the best form of free advertising: word of mouth recommendations. Your customers will recommend you to their friends if they are truly satisfied with your work, and your business will grow accordingly. You'll have get some customers coming back for repeat service and build up a consistent revenue pipeline.
Used-car salesmen unfortunately have some nasty stereotypes to live down, but you don’t need a tacky blazer or a fast-talking sales pitch to make money buying and reselling cars. People throw away perfectly good (or certainly good enough) cars all the time for all sorts of trivial issues. If you know a little something about servicing a car and can make one shine for a Craigslist ad, you could take advantage of massive opportunities to buy used cars and resell them for major profit.
A great way to come up with a business idea is to brainstorm about your existing skills. Focus on identifying the skills that are unique to you or that you particularly excel at. Evaluating your previous professional experiences should give you a hint. Think of instances in which you were able to provide value to the organization for which you worked: do you notice any trends?