Basically, the seller is selling a property that probably needs some TLC, and the seller appears to be willing finance (using a wrap [aka subject-to mortgage, lease-option [aka rent-to-own], etc) the purchase 100%. These types of contracts are straight-forward; a mortgage is a mortgage. The only difference here is that the seller--instead of a traditional lender--will finance the purchase.
This will be a win-win for both the buyer and seller, because the seller will receive more money, and will be able to sell the property sooner. The buyer won't need to qualify with another lender for the loan, and the buyer will most likely have to deposit less money upfront.