Tesla forbids any unauthorized party from even seeing a repair manual for the car, except where they're legally forced to by "right to repair" laws. (In those jurisdictions, they charge a high price for time-limited access.)

But, bear in mind, the difficulty in getting access to Tesla service or parts outside of the official methods is what makes the parts so valuable: There's almost none on the market to begin with. Those who need them (for whatever reason) will have to pay big to get them, since competition isn't driving down the prices. And most Tesla owners can obviously afford it.

There's a small, emerging aftermarket parts market, but it's mostly accessories at this point.

Ebay has quite a few parts so there is definitely a market out there.