The answer to this question is important to understand, otherwise this marketplace may seem attractive to lesser-scrupulous companies and scoundrels, and keep the legitimate players away.
They are different from dealers: you don't "buy" your Tesla at their store, it's for doing test rides. And they will walk you through filling out your purchase on the website. Part of this is semantic, part of it matters.
Those aren't Tesla dealers, those are Tesla stores.
Most (all other?) car manufacturers sell in the US via independently owned dealerships. Tesla sells directly to customers (where I am, at least) and they've had problems in some states because they have laws that prohibit direct sales by manufacturers.
Likely to avoid defeat attempts. The corporate email requirement acts as a first-line defense to at least try to stave-off would be skimmers / scammers grabbing the CAD file and working out the measurements needed to defeat it.
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