Just thought I'd provide my experience in case anyone is doing the same... basically on Ricardo and without going to the Strassenverksamt with the new owner.
I sold my car on Ricardo and in the end it was relatively easy. Now unlike when I bought the car off a friend (where we both went to the Strassenverksamt to swap plates, etc.), the guy I sold it to on Ricardo wanted to pick load the car onto his trailer without visiting the Strassenverksamt and registering it. There is no need... especially if he is selling this car on. So this is what I did:
Downloaded a blank sale receipt (kaufvertrag) from TCS'
website and filled it in with the details. I specified problems with the car and that there was no guarantee (sold as seen, etc.)--this is what I described in the Ricardo advert so the guy knew this already.
Went to the Strassenverksamt and had my fahrzeugausweis cancelled. They basically stamp the document leaving a big red "Ungüiltig" notice on it. They informed me that I can keep the number plates on the car for 14 days. This is so that the car can legally remain on the road and be driven to the next buyer if needed.
Guy came with his trailer and he signed my TCS sales receipt/kaufvertrag, handed over the dough, etc. I then took the plates off and he loaded the car onto the trailer. I later returned the plates to the central post office in Zurich (this could also be done at the Strassenverksamt of course but the post office is closer to me). The post office issued me with a confirmation document.
The next day I returned the city parking permit and sent a letter to my insurance company saying that I wanted to cancel my insurance and that I had returned the plates. I also gave them my bank details to both the parking office and my insurance company.
I am now waiting for refunds for the parking office, insurance company and the Strassenverksamt (for unused road tax for 2011).
The guy who bought the car will have to register it with the Strassenverksamt in his canton if he wants to put it back onto Swiss roads. Otherwise he can export it.
I was told by the Strassenverksamt that as the fahrzeugausweis has been cancelled, once I return the number plates I will no longer be responsible for the vehicle (of course I couldn't just leave it in the road, but basically I am not responsible for what the new owner does). In case there are any disputes as to ownership, I have a written document (in the form of a TCS sales receipt/kaufvertrag) showing that a transaction between me and the new owner took place.