According to Wikipedia, Mary of Antioch sold her claim to the Kingdom of Jerusalem to Charles of Anjou in 1277. Were sales of such titles common in the Middle Ages? If so, who would buy them - in this case, it was another ruling noble, but would a younger son or a wealthy non-nobles ever purchase a title for himself?
And finally, what authorities if any would regulate such a sale?
Great question. To build upon this, can we calculate how much certain titles might cost in today's money?
What this "sale" represents, is a portion of the top of the political leadership of "Frankish Outremar" (so called because of its ties to the French aristocracy) trying to find a political and dynastic settlement for the Kingdom of Jerusalem. Mary is not just selling to the highest bidder, but more trying to create legitimacy for a powerful figure that can support the claims of her family/faction members in the Holy Land. Charles of Anjou perhaps is trying to create a sense of legitimacy for his Mediterranean "crusades" rather than writing a check for purchasing an empty title. We should also note that neither party really obtained their aim as a result of the deal.
In short, this might be thought of as a clumsier than normal episode of medieval dynastic politics as usual, and not a separate pathway by which titles or political legitimacy could be obtained.