I think I'm over-engineering so I could use a second opinion. Let's assume that nothing off the shelf met your needs, so you had to build the website from scratch, what programming language, frameworks, databases, hosting platform, etc would you use to build your application.
Requirements:
* 500 daily active users
* 1,000 product listings
* Very low maintenance
Features
* Browse items for sale
* Search, filter products
* In app messaging (Buyers can message sellers, sellers can respond)
* CRUD interface for sellers listing products
* User authentication
It feels stupid asking the question, but it seems like none of the "modern" web frameworks fit such a simple use case.
Why is that? To me it looks like Django or Ruby On Rails would fit the bill perfectly.
The only things you'd need to do by yourself is the searching & filtering (possibly integrate with ElasticSearch since conventional databases aren't really good at search, especially not full-text search where you want a certain fuzziness to account for typos).
Authentication comes for free thanks to the framework, and Django even gives you the admin-side for free so you have a basic CRUD interface for adding/deleting products.