Folks, if you want a definite answer, take to a lawyer in your state.
As Nathan says, MOST states take the stance that once the Bill of Sale has been signed and money exchanges hands the "OWNERSHIP" and full responsibility of that ownership passes to the new owner--unless specific terms are spelled out in a contract--This is according to MY attoney and here in the state of Oklahoma (I'm not an attorney so don't take my word as gold).
My sales contract states that upon signing the sales/term contact the responsibility for the horse becomes that of the new owner--I also assume that on any horse I purchase--if there is a delay in transportation or I purchase on terms, then I make sure I protect my investment by insuring the animal until it is in my actual possession. Most of us are responsible horse owners, but there are things beyond our control that can happen.
I also allow 30 days free board for transportation arrangements, but that does not mean I'm responsible for any illness or injury to the horse as long I'm I being deligent and provideing proper care, custody and control.
If you look at any auction that is held, and read the fine print of the terms and agreements. The resposibility of the horse sold moves from seller to buyer when the gavel drops--therefore if a horse should go out of the ring-rear over and hit it's head on the concrete, gets loose and runs off, colics or gets injured in it's stall--it is the purchaser's responsibility.
It is best to check the laws in your state to be sure and if attending those auctions to read that small print in the front of the catalogue.
Another good idea for contracts, especially if selling on terms, to be sure you add a clause that states any legal matters will be ajudicated in the state that the horse was sold in (IE seller's place of residence). One thing I had not thought of and my attorney added to my contracts.