Will some communication (e-mails and probably the recordings of a mobile phone conversation) about the property details for negotiation of selling / buying of a landed property (in a rural area of Tamilnadu) amount to an agreement of Sale?
The negotiations may be agreed or not but they cannot be termed as an agreement. Agreement is totally different term to that of the negotiation.
Is it mandatory to reply to a lawyer notice?
It is always better to reply to a lawyer's notice atleast to record your protest/objection/denial of the contents in the said notice.
If a recipient of a lawyer notice himself can reply or that is it that the reply is to be given (on behalf of the recipient) only by a lawyer?
The recipient if understood the contents properly and knows what is to be replied and to give a non-committal reply, the reply can be given by the recipient itself.
Will the issue simply of a lawyer notice be prohibitive of selling a property by the recipient of the notice?
The lawyer's notice cannot restrict or prohibit the sale of the property, you can very well proceed with the proposed sale transaction.