Running into people selling a house they are under contract .

So it seems like a lot of property in my area has contracts on them. I'm not talking contracts that we think of when wholeselling. I'm talking where people buy a house under contract to pay the deed holder without having to get a traditional loan. For example, a guy is selling a rental property. He's notthe deed holder. He's the contract buyer but he rents it out rather than living in it. How would I go about dealing with something like that? I really don't want to buy if I'm paying what the contract holder is needing to get the deed from the deed holder.