Can garage sell a car thats not of of dealer franchise?

Can a main dealer on request sell a car that is not of his dealership? eg can I trade in a ford with Ford main dealer and ask dealer to obtain a BMW new car in exchange? If this is possibly where do i stand as regards warranty and car service?
I'm enquiring because imho the main dealer I bought from originally may offer me best trade in allowance.

Yes and there would be no warranty issues either.

Of course they will buy it at a trade price from a main dealer to sell to you. They have to do it this way to obtain your new car so the car you order may come from the main dealer you got a quote from in the first place..

Thank you.