What is the legal (Shariah) ruling on selling an apartment and what ensues from that, knowing that the seller was unaware of the absence of a proper electricity meter and that the building is in violation (of regulations)?

If the apartment has no electricity meter and one cannot be obtained, and the building has systemic violations, this is considered a defect that allows the buyer to annul the contract and return the item, even if the seller was unaware of the defect. The seller has the right to recourse against the original owner. However, a penalty clause merely for annulling the sale is not permissible according to Islamic law.