I may not be understanding correctly what was done to buy out the former owner. if you used the corporate funds to buy out the former shareholder the credit would have been to cash and the debit to treasury stock. This would not affect the basis you have in your stock.
if you use personal funds, that would seem to have nothing to do with the C-corp accounting. if you were not an owner previously your basis would be what you paid. This does not get recorded on the corporation's books.
seek professional avie to go over what was done.