A power of attorney is not an instrument of transfer in regard to any right, title or interest in an immovable property. The power of attorney is creation of an agency whereby the grantor authorizes the grantee to do certain acts specified therein, on behalf of grantor, which when executed will be binding on the grantor as if done by him. It is revocable or terminable at any time unless made irrevocable in a manner known to law. Even an irrevocable attorney does not have the effect of transferring title to the grantee. So, power of attorney does not convey ownership. An attorney holder may however execute a deed of conveyance in exercise of the power granted under the power of attorney and convey title on behalf of the grantor.
Now to some of your questions:
a) whether I as POA holder execute the sale agreement to the buyer on behalf of my wife. It is clarified that this property will be executed in favor of the prospective buyer by paying appropriate stamp fees & duly registering it in Karnataka or TN as the case may be.
Yes, as a POA you can very well execute a registered sale deed provided you are autorised to execute sale deed in respect of the property in question in the said POA registered deed.
b) whether the registered POA in Chennai is valid in Karnataka or I need to execute a new POA in Bengaluru for the same.
Yes, it is valid if it is a registered document.
c) Whether for executing a sale of these properties, one needs to execute a special power of attorney & not a general power of attorney? Again, is this Special power of attorney to be registered in both TN and Karnataka?
Whether general power of attorney or a special power of attorney, it shold have a mention about the property details which are to be transferred by sale or other means by executing a registered document to the transferee in the said document.