Upwork: This website offers a great marketplace for selling just about any professional service. You don't need a merchant account, website of your own or anything else for that matter. All you need to do is be able to provide a high-quality service at a reasonable price. But be informed, you will have to compete with many others that are constantly bidding on open jobs.
Find a profitable niche: We’ve talked about this a lot. But, where are you most comfortable. What niche do your skills, values, and interests intersect? Do you have 10 years of experience as a technical writer? Do you have long-standing PR relationships that’ll be invaluable in helping startups launch a successful crowdfunding campaign? Determine what makes your value unique, and lean heavily on showcasing that strength to your potential clients.