What happens if you sell your hdb after 70 years old?

It will depend on the amount you have in your CPF accounts and the year you turn 55. As you know, whatever sum of money that was deducted from your CPF OA to pay for your house, including HDB Grants, will be repaid back into your OA with interest accrued. Since you are on the old scheme called Retirement Sum Scheme, the Full Retirement Sum(FRS) is based on the year you turned 55(https://www.cpf.gov.sg/Members/Schemes/schemes/...) If your RA balance is below said FRS amount, the money will enter into RA automatically and monthly payouts would increase accordingly. On the other hand, if your RA balance is above that of FRS, the money in the CPF should remain in OA and can be withdrawn anytime:)