A marketplace of apps works when those apps fulfill a specific need (ie Shopify or Atlassian apps) or are specific to a platform (ie Chrome/IPhone). Why would anyone be interested in surfing a marketplace of random apps? Why wouldnt they just use Google to find the app they need?

Second, even if you are in a marketplace, you still need to promote it. Tons of apps are on the apple appstore. And they will get no downloads if you do zero marketing. App stores arent a substitute for marketing

Creating a marketplace is a big challenge. (Not building it, but getting people to care about it and use it.)

Who are the existing marketplaces that you can start with? (e.g. I don't know, but does Amazon have a "marketplace" for tried-and-tested Lambdas? And this just makes me think of Zapier anyway.)

I can name https://bit.cloud/components as an existing marketplace. It is a big challenge, yes, but also a big profit.