I copied this from a post on DS
Tara bought him a very expensive motorcycle as a way of trying to win him round after an argument.
Since he didn't appreciate being, as he saw it, bought off, Biff decided to sell the bike and use the proceeds to purchase Keepers Cottage which was being sold by Home Farm due to whomever was the owner then needing cash. I think there might have been a small mortgage in his name but Seth and Betty's wages were enough to cover that.
He then proceeded to go on living there with Seth and Betty - he'd been their lodger for years and viewed them as surrogate parents. He then said they could stay for life when he left.
So, he didn't exactly buy it for them, he bought it for all of them, using money he hadn't earned, didn't see as being his and didn't really want.