You Don't
Right up until the point when the contract is signed, the buyer can change his or her mind, and what the co-buyer thinks about it is an issue for the two of them to work out. After the contract is signed, it's pretty much too bad for the buyer if he or she has "second thoughts".
If your question is "can the contract be null and void" if signed only by the primary buyer and not by the co-signer? Depends. If the contract is in both names-yes. If the contract is typed up "only" in the buyer name-no. If the loan is conditioned to both signing, yes.
When it has been signed by the buyer and seller.
The date the Buyer signs it.
release the vehicle to the buyer
Find a buyer.
In Pennsylvania, a buyer's order only becomes consummated once the buyer has taken delivery of the vehicle. Not at the signing, but once the vehicle leaves the sellers property.
The contract is not enforceable unless both parties signed it. If the sellers changed their mind and didn't sign then you don't have a contract.
It will vary on the buyer and the copy.
That depends on whether you have signed a contract with the buyer.
yes
Not legally unless the buyer agrees to it.