If you sold your property mid year your taxes should be discussed with the Title Company.
At the closing, the buyer should have received a credit for the amount of tax the seller owed for the year.
You are now responsible for the payment of all of the taxes when they are due. If you believe that this does not apply in your case, contact the real estate company who sold you the property and have them look at the closing papers with you.
If you got a tax bill for a Real Estate that you no longer own, Please first check with the Title Company to see who is responsible for the taxes. Depending on when the property was sold or if it was split from another parcel it may determine the tax responsibility.