I recently purchased a house from Jewish owners. It is my understanding that the previous owner has to leave his Mezuzahs, instead the previous owner asked to be paid for the Mezuzahs. Am I obligated to pay him?

The previous owner is not allowed to leave the house without Mezuzot, but he is not required to relinquish his ownership. You are also obligated to put up Mezuzot. Therefore he can ask for payment. Normally the full price will not be asked since the Mezuzot are used. If you want you can buy new Mezuzot and give him his. But he should be reimbersed in some manner.