The lot I own is in Indiana. I let the property go for propert tax sale two years ago and a person won the bid but they still claim that I am the owner. The development where the property is located has charged excessive fees on the property and I want to abandon the property which I paid $ 8,500.00 for years ago. What is the best process to abandon the property?

If it was sheriff saled then you would not own the property and you should be able to find some documentation on that. Although they can still come after you for any difference from if you owed anything and the amount they received at the sale.
Any debt occurring once it was sold should not be your responsibility that I am aware of.

The rules on tax sales vary by state. In many I am used to, the buyer of the tax sale gets a tax certificate which they hold for a period of at least one year, and up to three in a few places. Then after that time, they can go to court and redeem the certificate for a tax deed.

The trouble is with debts that occur from the time of the sale till they redeem it.

But during the time before they redeem it, you are still the legal owner by deed.

You walk away, don't pay on it, and let someone take over ownership for some of the debts.

Did you pay outright for the property or finance it? Have you tried to sell it? If there is no outstanding loan, you can just not pay the taxes and/or fees and let the property be repossessed or sold for taxes. If you have documentation that this was done, then bring it to whomever is claiming you still owe anything.