Well... i believe that when one choose the flat, the seller (i.e. the current owner) got to be part of the decision making process as well. If the owner didn't disclose certain status, perhaps there is someway to pursue it further?
Anyway, I would think that pasting a big note outside your new flat saying that "Flat sold to new owner, please contact previous owner with phone xxxx if necessary (in chinese or hokkien, please).
What do you think?