1) You can't sell items that can't be sold. Even if you manage to add the "X" button with a memory editor it will say something with an error message and that is because they have the check on the server too.
2) There was an update that they updated the server with new scripts(When they updated so you can use 3 slots grenades on default) and in that update they enabled you the possibility to put a nickname of 12 characters length. Also I think it was the same update they've have let you to use those special characters.
Now it isn't allowed because they've reverted the server's files to the old ones(They had some problems with updating the game from that update or maybe it was intended). That's why you can still get the item "Additional grenade slot" occasionally. Even though there isn't any locked grenade slot.