Sale And Purchase Agreement Template

I'm going to test drive a car of tomorrow that is for sale on eBay. If all goes well I want to pay the seller a deposit after we negotiate the purchase price.
I want to protect my deposit and the purchase price through an agreement so that the seller does not take my deposit and sells the vehicle to someone else on ebay.
Are there any sales contract templates available that you could help me. The ones I found online do not seem to protect myself in this situation.
Thank you in advance for your help ...

I have not checked, but you can try nolo.com, they have a lot of free legal documents. Why mess with a deposit, if?