If you absolutely must give a deposit, use a service like escrow.com. Basically, you give the deposit money to them and you and the seller agree ahead of time what the parameters are for the money to be released to the seller.
I used them once when I helped the owner of the company I work for sell his $100k Porsche 911 Turbo. In that case, the buyer put the entire amount into escrow.com. Once escrow.com told us they had the $$, we shipped the car. Once the buyer received the car, he verified that the car was as advertised, and then escrow.com released the funds to us.
2008 E90 M3
2015 M3 (ED on May 26th, 2015)