I just got to the part where Columbo asks: "What about the ones you DON'T sell?" and you're absolutely right. He meant to say "DO sell" and I guess he flubbed it and the director thought: "Nobody will ever notice it. I'll just leave it in". Well, here we are half a century later and you noticed it. Nice work.

Btw, I always watched columbo in italian and I'm pretty sure this mistake isn't present there, once he asks about those they didn't sell and then asks about those they sold.