This will require the Storefront addon and SEO addon installed and in use to see the options and the affect.
Once done the seller's username will be used within the storefront url. It they do not like that and wish more control over that part of the url they can set their own as long as it's available. Have them go to their own storefront control panel and look to the following admin tool setting:
CUSTOMIZE > STOREFRONT NAME
They can insert a name they like. If that name is currently not used by another user that will become their new storefront name within the stores list and within the url of their storefront if the above is applied.