I'll start with your second first...
Cars depreciate the second they get to their first owner, servicing them merely slows the speed at which the value falls away.
As for him owning the parts
yes he paid for them, but he donated them to to the owner of the car when he fitted them.
Your wife owns an ipod. The headphones are a bit tatty, so, without asking, you by some nice new ones. A week later she sells the ipod and new headphones to a man in the market. By
your logic, you are entitled to demand the headphones back simply because you paid for them
To quote Vamps from a month or so ago...
Shall we just cut to the chase, you bought a car, paid cash and there is a problem with it, you have no choice but to live with your decision........