Thanks for your question to Shop4U theme.
To turn your site into the multi vendor website, you need to use the multi vendor plugin. Shop4U supports Dokan, WC Vendors, WC MarketPlace and WCFM Marketplace. They have free versions. And if you want to use the Pro version, you need to purchase directly from the developer.
If you want to add a new feature into your site - that is not included in the theme, you may also use a third party plugin and you need to pay for it.
For the cache plugin to improve the site, we might support you after purchasing the theme.