Earlier quoted context omitted.
This person isn't an investor, but even an investor can't sell their board seat. This is getting silly. "Hold on to your founder board seat and sell it later" is not real advice.
Depends on why you have the board seat.
The same is true of selling your equity, by the way. As a founder you have common shares, but early-stage investors want preferred shares with QSBS treatment. Even if you're allowed to sell your shares, which most startups don't let you do, it's not in your power to convert them to preferred or give the buyer QSBS treatment.