As per car manufacturing now days, most parts are manufactured by a third party. This usually comes down to who sells the parts at a lower price but also produces the parts to a quality expected by the car manufacturer.
What i am trying to say is that it would be good to have to order all the parts required for building the car (similar to Street Legal Racing)(Cam, Head Gaskets, Batteries, seats, pedals etc, however this would be done a bidding basis where you can negotiate bulk purchases, over time you could get loyalty bonus’s on new orders / Specs.
This would also allow for inventory control, stock monitoring etc. This would also allow for stock to run try and the production line coming to a stand still. This in real life costs car companies millions.
This would be a lot of detail to add, especially as all parts required will need to be put on the market place, however this could be an option that can be set to automatic if people down want to negotiate prices or play to business / strategy levels as other people.
If this is something that you would be interested in, I have a relative who works for a company who build engine pipes, tubes, hoses etc and could get a base rate price.