which language I should learn to build a platform like ebay

To create the front-end of the site, or the part that people see on their computer, you will need to learn a combination of HTML, CSS, and JavaScript. But for something like E-Bay, you will also need a language to create the backend of the site, so you can have users, store product data, etc.

There are several options for creating backends that are available:

• Java with Spring
• C# with .NET
• PHP (by itself or with Laravel)
• Python with Django or Flask
• Go
• JavaScript with NodeJS
• Ruby with Ruby on Rails

All these languages and frameworks have the features you need to create a backend, so it really comes down to preference. NodeJS might be nice because you are already learning JavaScript for the front-end. Ruby and Python and have syntax that is very easy to understand. Go is known for its blazing fast performance.

I personally use Swift with the Vapor framework, but that might not be a good choice if you are learning through Treehouse because they only have a crash course on it, that doesn't give you the foundation for building website backends.

Your goal would be a very challenging task even for a team of very experienced developers, not something I would think of as a good starting project for a single individual learning to program!

You might want to consider that you will eventually collaborate with others, and your own involvement might just be with a part of the total project. With that in mind, I'd suggest starting with learning the technology that you think you'd most enjoy working with (server or front end, design or development, etc).

If you have no preference (yet), perhaps the Front End Web Development track would be a good start. It will give you a good introduction to the 3 most important technologies for the client side (HTML, CSS, and JavaScript).

I don't agree that this project is so difficult that its even hard for a whole team of experienced developers. I think that it is possible for you to built a stripped down, bare-bones version of eBay that you could then add more features to over time. But in order to build this, it will be a lot of work. If you don't know them all ready, learn the basics of HTML, CSS and JavaScript, and then learn how to use a front-end framework like Bootstrap or Material Design because this will speed up your front-end design and development. Then, I would personally recommend learning PHP to build the server-side of your application, but C# with ASP.NET or JavaScript with Node.js and Express would probably also work for you.

Like I said, I think this is doable, but its going to be a massive amount of work. You will have to learn a lot and then spend possibly hundreds of hours designing and building out your site's core functionalities. If you feel passionate about coding, then this will be an extremely rewarding experience for you. But if you only want to build the site to earn money and you don't actually get excited about doing software development, it is going to be torture.

Good luck.