Assuming they are really there to buy, they'll have the cash with them. If they expect you to trust them with your Spyder, they have to trust you with their cash. I'd consider a short, simple agreement they sign that says if they damage the Spyder, they buy it at the full asking price. I'd also make sure your insurance covers you for whatever might happen, as it is your responsibility (at least in CA).
If they don't have the cash to purchase, they aren't real buyers, anyway. Make the policy clear before they come by.