Set up a multi vendor e-commerce platform like eBay, user can sell their own

This platform need to accommodate 5,000 users, who can list their own product to sell, also they can set up their own store on my platform, just like eBay, Auction function is also required.

I know a few open source ones.
Free open source platform is preferred. Need to be very robust and secure, users' privacy is of high priority.

Please let me know how long you need to set it up and running, and the estimated running cost.

I've already got the domain.

Thank you