I'm assuming you're going to cut the new property line in the middle of the two structures? You could also shift it two feet so the more permanent structure is 10 feet from the line and the older, less valuable structure is 6 feet from the line. The seller would most likely want to shift the burden to you so she'll want to keep 10 feet on her side of the line and you'll have the 6 foot set back. I still don't think it is an issue since it is a preexisting structure. I'd draw up the sales agreement and see what happens. You might want to have the land surveyed and make sure that the new boundary between the buildings is very clearly marked.