The challenge is that car dealerships can be brands used by larger groups too. Until recently (and still in OSM) “Bristol Street Motors” was a brand used by Vertu. The challenge for data consumers is to know when brand is the brand of the shop, and when it is something like brand:sales**
** a tag that doesn’t appear in OSM - sells, not much used, is for “type of product” not “brand of product”, so sells:brand (suggested above), might confuse, but brand:sales, also suggested there, would not.