Can a respondent sell the property while the summons are pending

If there's no restrain from court he can very well sell hence you file a petition to attach the property before judgment for security towards the suit amount.

can we petetion the sub registrar requesting to reject the sale of the said property 

Sub registrar has no authority to refuse registration of sale deed unless court's stay order so better get restrained him from court. 

The Sub Registrar cannot stop the registration of a document unless there is an order from the court or the government.

As per section 52 Transfer of property act the parties to the suit can't sell the property while the suit for deciding the ownership to such property is still pending in the court if the party does so the ownership of the buyer will be subject to the decision made by the court if you won at the appeal level and go for execution proceeding his ownership will be declared as frivolous and he would be required to leave property as such whether constructed or vacant and you can also seek demolition 

Moreover you can ask court to take the property in its custody so that the parties may not loose their interest.

There is no restrainment order passed by any Court, hence it can be sold.

Provision of Section 52 of the Transfer of Property Act is not applicable in the instant case.

Yes, X can sell unless there is a stay granted by the appellate court. If an ex-party stay the same should have been intimated to the respondent.

thank you very much one and all 

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