Try putting the jsonData in params as follows :
Ext.Ajax.request({
url: this.getBaseUrl() + webServiceUrl,
timeout: 240000,
method: httpMethod,
disableCaching: false,
useDefaultXhrHeader: false,
params: {
jsonData : {
"seller_id":seller_id,
"name":name,
"email":email,
"mobile":mobile,
"area":area,
"message":message },
}},
success: function(response){
var text = response.responseText;
// process server response here
}
});