When obtaining a power of attorney, the applicant must attend the notary with proper identification to the notary and give the full name of the person you want to give the power, which shall show details of the object for which you want to grant the power, which would be the floor sales and in which case it only needs to sign the grantor of the power.
With the simple act of exercising it, the agent accepts the power and presence is not required to sign or power.
When a developer sells a floor there is usually no problem, but it is a private business, the document must be signed by all owners, through empowered people.
If the seller has given a power of attorney to another person to sign this document, the attorney must file the certified copy of attorney, i.e., the original signed by the Notary does not serve a copy and the power must fully empower you to the sale, so it is important to check, by reading the power of attorney, the attorney is authorized to formalize the purchase without problems.
In contrast, when to sell an apartment without granting power of attorney, as can be done with third parties, for example with a real estate through a purchase agreement signed by the latter, the contract is void to engage with the buyer, the buyer, therefore, may require that the sale takes place, so that the sales contract to be valid must be ratified by the owner.
Without the existence of power of attorney that authorizes it to that end, the contract is void if not ratified by the owner who wants to sell. If you do not want to give power of attorney, it must of be a unity of action, shown on the scriptures in a single act buyers and sellers sign and passed the property.