An Ocean marketplace app is one of the primary ways that end users use the Ocean network. For example, a data scientist could use a marketplace app to see what data sets and data services (data assets) a marketplace has on offer. They can use the marketplace app to buy access to assets. Publishers make those data assets available.
[Pleuston](https://github.com/oceanprotocol/pleuston) is a reference marketplace app written in JavaScript, using React and squid-js (which is like a JavaScript SDK for Ocean).
Pleuston has an Apache v2 license, so you can fork it to make your own Ocean marketplace.
This option is straightforward because you can follow the dev process used by the Pleuston devs. They already have a Docker Compose setup that runs all the other Ocean components needed for testing a marketplace, e.g. Aquarius, a database for Aquarius, a local Parity Ethereum node, and Brizo.
There are many ways to create an ocean marketplace app. For example, you could use one of the existing e-commerce platforms and frameworks (e.g. WooCommerce, Magento, Shopify). Or you could use a lower-level framework like Django or Vue.js. The main consideration is that you should probably use a programming language with an existing Squid library.