Taking a small deposit to 'hold' a vehicle is fairly typical and it's usually pretty small. I've made deposits of $200 on $50,000 vehicles. It's also common to return the deposit if the buyer quickly changes their mind, usually after they have done research on the model. Ask any consignment dealer. Now, if the buyer wants you to hold the unit for weeks, that's a different story...and you keep the deposit.
So in the real world, a deposit on a vehicle is rather informal. Don't over do it or the buyer might just walk...
As far as the sale goes, the seller hangs onto the title until all monies are transferred.
The more money involved, the stricter the informal rules become to protect either/both parties.
