If the seller wants cash only, there are several good reasons why.
1. They don't have the time to wait for a buyer to get a mortgage. They need cash fast.
2. The property has major condition issues that would require fixing before a bank would give someone a mortgage, and the seller doesn't have money to make the repairs.
I once had power of attorney for someone who had a property that needed to be sold and closed quickly, the property was in need of major work and was on the verge of being condemned by the town. A fast, all-cash sale was needed and that's how it was sold. The buyer had a major issue with a collapsing foundation wall and did not see the profit they expected to. The house was sold as is, with no guarantees. It didn't sell for a lot of money, but if didn't sell when it did and the town condemned it, it would have cost money to raze it and haul away the debris, more than what the empty lot was worth.