If you have a lien on your vehicle, you can sell it for any amount. The vehicle is still subject to the lien which means that the purchaser can lose the vehicle if the lien holder collects on it.

Sold as is means you are not responsible. You are selling it as it is.

Unfortunately in California there is no AS-IS clause for vehicle sales. If it is registered as operational and you sold as operational then the seller is responsible for smogging it. The title of an operational registered vehicle can NOT be transfered without a smog certificate. If it does not pass smog, the purchaser can require the seller to pay for repairs to pass smog.

The work around is to register the vehicle as Planed Non-operation (PNO) and sell the vehicle as non-operational. It may not be driven or parked on a public street and would need to be towed from it's residence by the purchaser. The purchaser would then not be able to drive or operate the vehicle on public roads until they registered it as operational. The purchaser would be responsible at that point for smogging the vehicle since they would be the title holder at that time.

Vehicle registration numbers are a means of determining the owner of a vehicle. Usually, when purchasing a vehicle from a dealer, the dealer will transfer the registration number to the purchaser and fill out the required forms.

A co-purchaser is a co-owner.

Assuming you are referring to the Bill of Sale, then the purchaser(s) of the vehicle should be the ones named with the lender being the lienholder until the vehicle is paid for.