choosing the right theme

I have no slightest idea on building a web page where individual can sell and buy items. I want a kind of a situation whereby individuals can subscribe and (for free initially) and place various items on my site (marketplace) and interested buyers of such goods and services can freely contact the seller and initiate buying process. A seller can only subscribe and place a limited item with pictures of an item and contact telephone number and probably initiate a kind of negotiation that will eventually end up a successful sale.
By the time the web site would have gained popularity, a token would be charged for such placement.
All sorts of legitimate goods and services would be allowed to be placed when a potential seller would have subscribed/registered with valid email and phone number.
what’s the reasonably good theme (Elementor compatible and something I can study and use without necessarily employ anybody to do initially) will be good for such idea?
Private conversation/chat to be enabled on the site.
As time goes on individuals can now be charged a particular amount to register and be able to sell with a token yearly renewal. Individuals too can create/develop editable (with some sort of limitations) market place within the site for personal store that will show type of item (new, refurbished or used)

This is not something that you can realistically handle as a novice, nor using a stock theme.

Aside from the technical considerations of creating this, this ongoing maintenance, security, hosting and other costs would be huge and very complex

If you have a sensible budget then you might be able to find a more experienced developer to help via, but even then I think you need to invest a lot more time and investment in planning this properly and exploring the wider considerations beyond just a theme to use.

can I be guided properly on how to go about it?

It’s too complicated without you finding someone dedicated to working on it with you -

There are so many other considerations and costs you need to think about - just some (nowhere near all) -

  • Decent and secure enough hosting (monthly cost)

  • Security (costs/dev/maintenance)

  • Site updates and maintenance. If you are new then how will you fix bugs or issues that users have?

  • Legal advice - you need to 100% sure you are protected if something goes wrong as a result of your website introducing people and/or purchases

  • how to handle membership and user data

I appreciate you want to start small and grow but a lot of this is the very basics, and as/when/if it gets even slightly bigger, then done properly you will be looking at hundreds if not thousands of $ a month to maintain it

Can I have an idea of a budget so that I can properly play