The larger context is that what they bought was my entire business, along with all rights to the intellectual property. What I got was a sale price and defined ongoing royalties for the product I formed the company to create.
They did sell something they owned. I neglected to attach a method by which I'd get compensated for them selling that particular thing.
In the mindset of cutthroat business, they legitimately won. They 100% adhered to the terms of the sale, and it's not their fault that I left a loophole they could leverage.
My attorney did say that if I wanted to, a case could be made for a lawsuit -- but it would have been expensive and wouldn't have had high odds of success. I just wanted to move on.
This whole event was my first real business success, and the mistakes I made were legion. It taught me quite a lot -- including that I won't do business with anyone that I am nervous about doing business with. Contracts can only protect you so much.
Also, I feel the need to repeat... I did pretty well from this deal. I was angry when this happened, but with the passage of time, I see that even with this event, I came out of the deal better than I went into it. So I hesitate to even call it a "regret". It's more of a "learning experience".