Can I sue the landowner for foreclosing on the land I purchased as owner financed.

The simple answer is yes, the previous owner violated your rights by using your property to secure a loan.  However, this situation doesn't smell right to me.  When the original owner sold to you, you, in the normal course, would have recorded that deed.  That would have shown up on any title search, and no lender would thereafter have loaned money to the previous owner secured by that property.  Or are you saying that there was another loan secured by the property BEFORE you bought it.  If so, why were you not aware of the loan?  Again, it would have appeared in the title search. Somewhere there's an important piece of the puzzle that I'm not getting.