Hate to disgree with you..but that is not true.
All you need is $1.00 to make a contract binding.
I know - which is why I said "whatever amount both parties agree on" ... but stock standard is 10%, which most vendors (only buying/selling 1-2 times in their life) would insist on.
I seriously doubt many vendors, and their solicitors, would accept a deposit of less than 5% because if you don't complete settlement then they have to chase you for the balance of your foregone 10%.