There is the 5 year MOP period (from TOP Date) stipulated by HDB that owners of ECs can only start selling their property in the open market. But sell to whom? For those who aren’t sure, it can be sold to Singapore Citizens (SC) or Singapore Permanent Residents (SPR) after the 5 year hiatus.
From the 10th year onwards, you can sell to anyone including foreigners.
Disclaimer: The above information is for reference only. Please confirm with HDB or your law firm if you have any queries.