Difference

You should have demat account to credit shares in electronic form from physical custody. Certain documents needs to be signed and given  but not Power of Attorney for sell or purchase by him.

If you want to dispose off all the shares held by you and do not intend to buy any company's share for the present even for investment purpose, you do not have to open a trading account with the broker neither you need to give power of attorney to the broker for this purpose.  If this broker insists on that, you may look for a different broker, all that thy need to do is to convey your sales for which they will be getting their due commissions out of the sale proceeds for which you may have to give certain forms duly signed, before signing any form please read them carefully and then sign it. You can approach a genuine and reputed broking company for solving this issue.