if selller has to give power of attorney it has to be to its close relative only . it should be his family member . the said power of attorney should be registered . only then can seller through his POA sell the flat to you . in such transactions it is better to play safe . contact a local lawyer . get POA drfated by him and get it duly stamped and registered .
affidavits wont serve the purpose , either seller should sign sale deed or his registered POA holder . thye poA can appear for registration purposes if he is duly authorised