When I get my car fixed at the mechanic, I am paying for the service but at no time do I transfer ownership to him. One procure we could follow is for him to do the repair, give me my car back, ask for payment, and then pursue me in court if I refuse to pay. But instead, the procedure is that he just keeps the car in his garage until I pay. (If I want to call the police to get my car back, I can, but now I have to explain to them why I'm not paying, and enforcement is much easier.) This forces me to respect the agreement we made - repairs in exchange for money - in a more robust way than any contract ever could.