Hello there, I am looking at a car for sale however the seller says there's no grey card (also the car is not in running shape, requires work). I'm not too familiar with the process, is it simple to get or is it super complicated?
Thank you and have a nice day!

Quick answer: walk away.

Could be any of a few scenarios:

1. Seller is legal registered owner but has lost the grey card. He can simply apply for a new one, takes just a few days. Unlikely, otherwise would have already done so.
2. The car is stolen. Say no more.
3. The car is not registered in CH. Probably means it was never officially imported, so you'd need to do that, pay the VAT on it, then get it tested before you'd be issued Swiss documents and plates. Complicated, yes.
4. Anything else where it's been passed on, legally or otherwise, without papers, again unlikely, as Why? You'd need to get the VIN and check with the police that neither 2 nor 3 apply, and what you would need in order to satisfy them that you are entitled to register it in your name.

Agree with previous poster. Do not touch with a barge pole. If you really want the car, tell the seller to get a replacement registration document. It won't cost that much (will depend on the canton).