A question about single player

Can you use the same cars you can use online?If you can,but there is a different way to do that,how do you get them?

Yes, all the cars you have in single player are the same ones you use in multiplayer.

If i were to only play single player could i get all the cars acessable in multiplayer,could i buy other people s cars,use other people s tunes,could i use other people s paint jobs?

Answerinng you questions one by one, I may be wrong :
Depends of the car, most of the times yes.
Yes on PC, yes on Xbox if you have Gold Live membership.
Yes no matter what.
Yes no matter what again.

You can’t easily get all the cars solo, it’s harder. You can either try to win the Vinyl comp, win the photo comp, look in the auction, or wait for the cars to come out for everyone.

Thanks for the information,I appreciate it