How would that work Ghost?
Your alt account will simply go through the same process as those not yours and still get the engine.
Also why is it an issue? If I manufacture an engine, I'm doing it for myself and nobody else.
You can exploit the fact that you earn money from the costumers. Once at level 30 you can create multiple accounts to buy the engines at max cost and farm money.
They can check IPs to see if the accounts belong to the same person. Of course it's not perfect and it can be a person from the same household but distinguishing this cases shouldn't be a problem. Enforcing then penalties such as account suspension or permanent ban can discourage this behavior.
I do agree that the contract being free to however gets the link is bad.
The best way, in my opinion, is for customers to make an offer and the manufacturer can either decline or accept.
This way it reflects more on what an engine manufacturer would do, sell the product at the higher price possible.
You are right on the money exploitation and it could add up to a huge amount. Hopefully something to limit that can be done, though I'm doubtful the IP route is the way.
A big YES to manufacturer/customer contracts.