IS A SALE AGREEMENT ON PLAIN PAPER NOT NOTARISED A VAILD LEGAL PAPER

I'm not sure exactly what you mean by "valid legal paper", but if you're asking whether the fact that it is not on some kind of "special" paper, or notarized, means that it is not a valid contract, the answer is probably not.  Many types of contracts are not required to be in writing, and even fewer are required to be notarized.  There is generally more formality required in instruments such as wills and deeds, however.