If you have tried and tried to find the seller, but have had no luck, you last option is to hire a professional. There are many skip tracers out there who are very good at finding people. The best company I have come across is findtheseller.com.
A skip trace is a comprehensive search that will help you locate “lost” people. Finding the seller of a vacant home could potentially make you a lot of money and the rates are reasonable, so check it out if the other options haven’t worked.