You need a WordPress host that supports WooCommerce. While free themes exist, free hosting does not. Use a low-cost provider like (pay-as-you-go) or SiteGround (StartUp plan – ~$3/month).
A: No. All themes listed (Storefront, Blocksy, Astra) work via the WordPress Customizer. However, knowing CSS helps to tweak vendor list spacing. multi vendor marketplace wordpress theme free
The moral? A free multi-vendor marketplace is entirely possible, but your most valuable currency won't be money—it will be the time you spend learning how to glue the pieces together. You need a WordPress host that supports WooCommerce