What if the HDB flat is sold after signing a contract with a retailer. Will there be penalties for this early termination? Or what would be the various possibilities?

It would depends on the terms and conditions of individual retailer and that if any allowance to transfer of the contract to the new address due to selling of old flat is possible a not.

Also if there is any additional charges and requirements needed for transfer of contract to the new address.

At the time of signing up with the retailer is it done online yourself or with a representative . if with a representative have you thus been inform or have been clarified before allowing you to proceed with the signing of the contract.