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.
The Field Agent app is available on Android and Apple devices. You only need to look for tasks within your area, do the research, submit the details of your findings and wait for your money. These tasks are simple and involve visiting stores and checking on issues such as display compliance, demos, and shelf availability. Each job may give you earnings between $3 to $12.