@britainsfarm.co.uk, In order to list item, a vendor must register to the site via Vendor Registration form.
Only after becoming a vendor, they can add product. As you can understand, a user need to register for vendor only once. After that they can simply log into your site and add any product.
For further clarification, please check our demo and see the flow : https://multivendordemo.com/mvx/my-account/
If you have any modification in mind, feel free to share so we can assist you with this.