new owner

I am about to buy a vacant lot. The owners of the adjoining lot unknowingly placed a small shed over the property line some 15 years ago and want to leave it(it wouldn't survive being moved). I wish to allow them to leave it there but fear liability should they or one of their children get hurt while in or near the building. The current seller had a good relationship with neighbors, but I beat them out on bidding for this lot. What legal rights do I have? Can I post a no trespassing sign at the building and rest easy? Does the seller bear any responsibility.