Validity of Sale-cum-General Power of Attorney holder to sell plots (Vijayanagaram, Andhra Pradesh)

Sir, A venture is being sold by X in name of X as plotted development, with website and pamphlets. But the link documents show Y is the owner with acquisition of land papers through Sale Deed. When we enquired why X was selling the project as their own, while the documents show Y as the actual owner of the land, they say X & Y were partners earlier and got separated with individual Company Names i.e. X & Y and Y has conferred Sale-cum-GPA rights on X. Now, While registering the sold plots in the Sub-Registrar office, X is executing a - Sale Deed - with stamp duty, registration fees. Sir, i read that the Sale-cum-GPA itself does not confer powers to sell. My humble queries are:- 1) is the Sale Deed executed by X, and Sale-cum-GPA holder valid. 2) In case i buy, can i sell the plot to a third party, as from what i see, i my am a third party here. 3) Say if i pursue X to summon Y to sign the sale deed, is it valid, as Y himself has already executed Sale-cum-GPA in favour of X and other suggestions your goodself want to give. Thanks & Regards.