From what I learned when buying a house, property tax should be settled at the time of title transfer (closing). Since this tax is often paid every year or six-month and the transaction may happen at any time, the buyer often reimburses the seller at the closing.
Property tax lien goes with the property, not the owner, at least in my state. This is the ground that the county collects tax from you, because you are ultimately resposible for the lien if the county could not recover the tax. Usually the title search should reveal the unpaid tax. If it faild to do so, title insurance comes into play.
Here is answer to a similar question:
http://www.city-data.com/forum/foreclosures-short-sales-reos/456724-unpaid-property-taxes-reo.html
