Creating a vendor marketplace

We are searching for a developer with rich experience in creating a high-performance multivendor marketplace based on an open-source CMS. The shop should be able to handle more than100.000 items and a lot of sellers at high speed.

Just installing and simple modification of the theme is not enough. We would need a costume theme, SEO, and speed optimization of the website.
So backend and front advanced skills are required for costume solutions.

Please just experienced developers! Otherwise wasting both sides just their time.