Is it possible to build a marketplace app in Python?

Generally speaking, yes. Python is a general purpose language so it can be used to do whatever, but that doesn't mean it's the best tool to do so.

Ultimately you will need to include JS into the process anyway though if you want anything resembling a modern web app unless you plan to use WASM (but even then, you need JS -- and I wouldn't choose python for a WASM app when things like rust exist lol)

Yes, you can build a marketplace app using python by using Django (or any other python frameworks, but Django is my preferably one), which is a web framework based on python. You can integrate any web tool with Django as well as JS and its libraries and frameworks.
djangoproject.com/

I've worked on Django and Django Oscar before. The latter is a package which adds ecommerce abilities to Django.

Using something like one of those helps you get started with users and admin view and items or products and a well known and document path to integrate with Stripe etc. So you don't have to reinvent it all.

Django apps include HTML and CSS and JS for you. You could even go for Flask if you want more control or less functionality (like don't need complex shipping and multi country support).

If you are comfortable with Node.js or PHP then go ahead and use one of those. The company where I work includes an online store in Ruby on Rails , a popular flow in Ruby development. See what frameworks and themes exist for those so they come with the frontend look and feel and ecommerce functionality you need. See what tools are available and see how far you get spending a bit of time on each before you make a call on language and library.

Maybe you write our API in Python and your frontend in JS. And one day you can swap your API to be in Go or PHP and your frontend will work the same.

A reason for choosing JS would be if you want highly interactive behavior without writing tedious stuff yourself. React or Vue in Node.js would be nice here, but you'd still have to pick a backend whether is just plain Firebase, or a Node/PY/PHP API with a database behind it.

I am a fan of Python for its style and ease of writing and find fewer wild surprises of the language. So I think Python would be fast to develop in assuming you are skilled in it. Django can be overwhelming as well even if you know Python and ecommerce business logic can be tough, so expect a learning curve with whatever route you go.

Shuup, saleor multivendor module are based on django