If you were at a shop you'd NEVER say those words out loud
Wrong, I've done this. Because it's not a scam and I have zero moral reservations about treating the people like adults who can decide what sorts of trades are in their interest or not and if someone wants to give me shit for this I'm perfectly comfortable throwing it right back at them. If you don't want to trade with me, then fine blacklist me - see if you can find someone else that will trade you ZEN fetches for your WAR rares without a premium. But people who act like I'm doing something wrong simply for expecting it to be worthwhile for me to trade are just being narrow-minded. You don't expect to throw down cards on both sides of a trade and say "values are equal, you have to trade me" - your partner may have all sorts of reservations, and it's not wrong for them to ask for additional value on your end rather than just rejecting the trade.
I do find it amusing though that the 30% ask has been picked up by other traders.