That is why we leave you the ability to warn others that you will reuse the design, because yes, you came up with it. Regardless of that, you should realize that the design is no longer yours if you're selling it — it's like saying the pet is still partially yours. If you're discarding the design, you obviously don't have a strong attachment to it as it is, and that's why you're planning to redesign it in the first place. Like we say in the rule, we don't confiscate pets because you used the same shade of blue, and each case is unique.
Not to mention, if you did clarify that you were going to redesign them in your trade topic, then that's proof enough. It's written clearly, so people couldn't miss it when they saw the Cain for trade.