Is the POA registered ? 

2) specific POA should be executed authorising B to sell the property 

3) sale deed can be executed by B as both vendor and joint vendee

1. If there is valid power of attorney in favor of B than in that case B may sign as both seller and buyer.

Yes sale deed needs to be executed in above issue. Gpa can't be a supplement to sale deed

No, both person B and C have to sign

There is no legal infirmity for a power of attorney holder to sell the property unto himself as a vendor.

As a power agent he is representing the owner of the property and selling the property on behalf of the owner to  buyer on the basis of the power of attorney deed issued to him by his principal.

Thus he sells the property to himself on behalf the principal/owner, therefore, in my opinion, there is no legal infirmity in this act. 

1. B can sell the property for and on behalf of A if he holds a GPA which has not been revoked.

2. B can execute the sale deed as agent of A in favour of himself and C as joint vendees.

1. Yes, the person 'B' can sign in the sale deed as both Vendor and joint Vendee, based on the registered POA in his favour.

2.  It's a legally valid and binding arrangement.

No.

In contract of agency which deals with Power of Attorney as well the constituted attorney can not do any act which is considered to be an act of conflict of interest. 

In sale since the buyer always remains interested in lessening the price and seller knows only how to increase the sale price , B can not sell himself in the capacity of agent of actual owner. 

This sale would be avoidable. 

No this sale deed is not valid at any cost which is made by the "B" who got the property from "A" on GPOA  basis. kindly check the SC citations regarding this and secondly how the registrar registered the property of the sale deed on POA basis. there should be proper sale deed transaction or tripartite transaction of the sale deed. or B should have development agreement registered.

1. YES .... PROVIDED the POA is a registered POA with clauses to Buy & Sell.

2. IF above is true, THEN B can sell it to himself without any legal hitch.

- As per law, GPA is executed by someone who cannot perform his transactions himself or herself personally due to numerous reasons, and delegate his powers to perform a duty to some other persons,
- The reason may be , the Principal resides in a foreign country and due to that he is unable to be present for the responsibility, or is bedridden,or a senior citizen who is suffering with health problems or any other valid reason on the part of the Principal for being unable to perform his duties on his own.

- Since the person B is a blood relation , hence after getting the GPA , he can transfer the said property on behalf of person A legally. 

- As the sale deed was made by the person B , then this is legally valid .

- But person B cannot enter into an agreement on the ground of said GPA getting from person A , himself even jointly with person C. Such deed would not be valid document legally. 

Dear Sir,

As per the law in vouge, the same person can't be vendor as well as vendee. To come to a logical solution, B can gift some portion of that property to C and then they may enjoy/use the property jointly. 

If POA registered and In favor of B then B Can execute sale deed. 

Yes sales deed can be signed by B at place of vendor and Vendee where it is will be specifically mentioned as POA of Owner in Vendor.

B was a buyer ie vendee originally. Now PoA has been given to B. Please state clearly as to how he may become again a vendee ie seller.

When B was a buyer why is he being given PoA??

This cannot be done.