[Logician]

Edit member.php:

Find:
-- cut--
// ############################### start mail member ###############################
if ($action=="mailform") {
include("./global.php");
$userid=verifyid("user",$userid);

if (!$bbuserinfo['userid'] or $bbuserinfo['usergroupid']==3) {
-- cut--

Replace LAST LINE as
-- cut --
if (!$bbuserinfo['userid'] or $bbuserinfo['usergroupid']==3 or $bbuserinfo['usergroupid']==X) {
-- cut --

Change X to your Seller Group ID.

Enjoy..

Logician