What are the risks associated with selling life insurance to 3rd party - instead of simply surrendering it with its insurer/ insurance company?

No issues. They'll meet you at your insurer's office, get the paperwork done right there and there, and pay you on the spot. https://insurancejobs.sg/sell-your-insurance-po...

You will need to check with the 3rd party brokers if they will be willing to take over a whole life policy; believe they are more keen on endowment and ILP products.

That said, I don't believe that there are any risks involved with selling your policy to a 3rd party broker; I did that for my own endowment plan some years back. You will usually receive more from selling your policy than surrendering it with the insurer.