If someone's selling their car and the title is already signed but by the previous owner, is that a problem?

No. That shouldn't be a problem. The (current) seller can sign in the dealership line if necessary.

Title has to be in seller name and signed by seller on back to buyer and buyer signs. Put exact mileage and not guess a lower # than the last owner. Write everything with No mistakes, cross out or change the letter/# They will give a problem unless you get Lucky somehow. Good Luck