I'm unclear about the interest. How do I earn interest by paying off the tax?
The lien on the house assures that when the owners or the next buyers pay the tax bill, the county will give you the money. The county interest is to get the tax money. When you bought the lien, they got their money. When the owners pay off the lien and the interest, the county gives you the money.
When you don't pay your property tax there is interest charged. Sometimes for up to five years before the county auctions the property or awards it to the lien holder.
The county settles for the tax debt and gives you the interest. However, you may have to pay more than 1 year's tax bill to maintain your position to receive the property at the end of the process.
Remember some counties auction the property and don't award it to the lien holder, just their money.
Best wishes!