See if they want to buy more on the regular. Set up a supply line or discuss opening a gallery with this person. Why would a professional artist be upset ? It wasn’t a gift, it was sold.

My contract is written so they can’t do that. Lawyer up and take MY profits

I’d feel honored! Seriously. People buy for their own collection and some buy to resell, some may decide later to resell if they no longer choose to keep it. Ha! I’d see what they are selling my art for and possibly rethink my prices 😉