I purchased a property and have the deed recorded. It still has the prior owner on site and he says he can get the property back after about 5 mos?

Purchased real estate from a property reseller. The reseller listed the property as having current renters paying $350/mo. rent. The seller purchased it at a tax sale. It was months before the property was finally recorded and I could take possession. When I visited the property, the people living there indicated the male was the prior owner and had never paid rent or been in any leases. He also told me that he had already talked to an attorney and was told the he had about a 90 percent chance of getting his property back. He said that he asked when he had to pay the past due taxes and was told that they didn't have to be paid right away. Then he said that his property was sold without receiving a notification. I've seen to serve a notice to quit in person but does the owner have to serve it or can it be someone else? Can it be taped to the door if no one is home? Tried cash for keys, not interested. What are his chances of getting it back?