Consult with an attorney in your state to learn the maximum interest rate allowed for this type of debt.
Also, charge-off is not used in this context commonly, so I am unsure what you mean when you use that term. I suspect you mean foreclosure. If my theory is correct, then you should be working with an attorney on this matter to be certain you preserve your rights and protect yourself from potential liabilities.
However, if by your question, "What interest do I have in the property?" you mean, "What are my rights to the properties where I purchased the property tax liens?" then that, too, is a question for an attorney in your state. A person (or government) that owns a property tax lien has the right to foreclose on the property in some states. Again, this is a question you should ask an attorney, who will be able to review your documentation, ask probing questions, and advise you precisely.
I hope this information helps you Find. Learn & Save.