Conquest changes what people sell. Check Region info, and see who's in first. They will get everything listed the merchant sells. Bastok is generally not in first place.
I went to try and buy this from Zemedars in the Bastok mines at location H-6 and he didn't sell any leather armor. Why I have no clue, leather was a common armor in all the final fantasy games I believe, so I have no idea why its listed here as being sold by merchants when its not. The only solution I could come up with is its a mistake, or they do sell armor in limited quantites and REAL high levels like to buy them all out and then sell them at jacked up prices cause I could see greedy players doing that. Can someone tell me if this is true? Does any merchant sell leather armor?