When you order a free basic custom store and complete the survey, you will be provided a couple of options for your domain name that suit your niche. If you already have your own domain name that you want to use on this site, please let us know about it as soon as possible, as we will need you to set the NS records for your domain to our servers so we can work with it.
If you have an already running webstore and decide to change your domain name, note that you can face some issues with customization after transferring your store to another domain. So, you should be prepared to customize your site again. Anyway, you should contact Sellvia support team to inform us about domain switching.