For the one interested, here is the solution :
in user-non-secure.php, after line :
osc_run_hook('hook_email_contact_user', Params::getParam('id'), Params::getParam('yourEmail'), Params::getParam('yourName'), Params::getParam('phoneNumber'), Params::getParam('message'));
add the line
osc_add_flash_ok_message( _m("We've just sent an e-mail to the seller") ) ;