Remember that they'll get 3 candy for the trade as well.
Assuming it doesn't affect the cost, I'd be thrilled to trade a mediocre level 20 kyogre for a Mr. Mime + 3 kyogre candies! Not sure if you think that's worth receiving though.
I wouldn't want to trade a shadow claw gengar. I'd be more than willing to trade a weak legacy like my Twister Gyarados. Probably not interested in trading any of my body slam Snorlax though.