Purchasing OTC tax liens can be fun but here are two examples of how to lose money when a purchased tax lien is redeemed.
Mohave and Maricopa Counties in Az both charge administrative fees that must be paid by the purchaser of a tax lien. I was unaware of these administrative fees and it resulted in a negative return....always read the find print!!!
Examples:
1)Mohave County--tax lien purchased for $384.40 at 12%, earned $14.78 in interest and had to pay $10 processing fee and $10 auction fee....net loss $5.22
2) Maricopa County--tax lien purchased for $35.67 at 10% earned $.28 in interest and had to pay a $5 processing fee .....net loss $4.72

I don't understand why the tax lien purchaser has to pay for the administration fees. So read the fine print for each county you want to purchase a tax lien in and understand what fees have to be paid and who has to pay them :)