/run SendChatMessage(format("WTB all your %s for 100g per one, please /w me or C.O.D. me, thanks.",select(2,GetItemInfo(74248))),CHANNEL,nil,GetChannelName("Trade - City"))
This should be what you are looking for.
%s will be replaced with the item link based on the "GetItemInfo" call. The number after the "GetItemInfo" is the game id for a sha crystal. You can get game ids from WoWhead
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If you want an actual working/clickable link in your macro, you have to get more technical :P
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My trade is /4 >.>