Hypothetically speaking, let's say there's a chap that'd like to have the title of Duke of say... Manhattan.
Off the bat, I know that there are three options to purchase the dukedom title of Manhattan - those being the Gallic Royalty (who are trying to take over all of Sirius), the Rheinlanders (who believe that they might be able to take over Liberty), and the Libertonians (who actually own Manhattan)
This chap would want the title just for the title itself (for all those Doctor Who fans), with no benefits or perks other than to be able to be called by the title.
Good luck getting people to agree to this sort of RP though. (In reality I doubt people would mind too much if you did not adversely affect RP, and role-played a Senator with the unofficial title of Duke.)
If on the forums, and you mean user title, you need 2001 posts before you can change your title.
Liberty wouldn't work for this, but Bretonia and Gallia both have Monarchies, so you could get the title of Duke in either, if your roleplay fits. Unfortunately for you, "buying a title" doesn't fit...
Nobody buys a title, they inherit it or earn it. Also, the only way to get it would be to be Bretonian, or Gallic, as foreigners are very very rarely given these titles except through marriages.