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.
Complete errands or tasks for the elderly. Older people often need help with buying groceries, cleaning their home, performing home maintenance, and paying bills. To find clients, contact your local community center or church to find out if anyone needs help. Additionally, you might post an ad in your local classifieds or talk to people you know to find out if they know someone who needs help.