Next.js is a React framework that is popularly used by web developers because of its ability to build high-performance web applications and features such as server-side rendering and dynamic routing. If you are a developer who has experience with Next.js, you can leverage this expertise by selling the code you create. Here are some effective ways to sell Next.js code and increase your income.

1. Selling Codes on the Marketplace
   Platforms like CodeCanyon and ThemeForest allow you to sell Next.js code as templates or complete projects. You can create web applications, e-commerce, landing pages, or Next.js components that can be reused by other developers. The advantage of selling on a marketplace is that there is a large user base who is actively looking for digital products.

2. Using GitHub Sponsors
   If you're working on an open-source project with Next.js, you can get support through GitHub Sponsors. This is a way to receive financial contributions from the community that uses your project. The more popular and useful your project is, the greater the chances of getting sponsors.

3. Build Your Own Selling Website
   Apart from depending on the marketplace, you can build your own website to sell Next.js code. Using the framework itself, you can create a personalized platform to offer various projects, customization services, or even consulting. By building your own website, you have complete control over branding, pricing, and marketing strategy.

4. Leveraging Gumroad for Digital Products
   Gumroad is a very user-friendly platform for selling digital products, including Next.js code and templates. You can set up a simple sales page and offer your products at competitive prices. The advantage of using Gumroad is that the interface is easy to understand, it doesn't require complicated integrations, and you can start selling products in minutes.

5. Marketing Strategies to Increase Sales
   Write Blogs and Documentation: Create detailed blog articles or documentation about the code you're selling. This can help improve SEO (Search Engine Optimization) and attract more potential buyers.
   Provide Discounts or Bundles: Offering discounts to first-time buyers or product bundles can be an attractive strategy to acquire early customers.
   Leverage Social Media and Developer Communities: Being active on social media platforms and developer communities like Twitter, Reddit, or LinkedIn can help you promote your Next.js product to a wider audience.

6. Conclusion
   Selling Next.js code can be a profitable source of income if you have the right strategy. Take advantage of popular marketplaces, create your own sales website, and make sure you promote your code in the developer community. With consistent effort, you can increase your income through selling Next.js code.

This article can be developed further as needed, including adding example images, links, or code examples if necessary.