I don't think you did anything wrong. Every fish anyone ever buys is always buyer beware, and should always be quarantined by the buyer anyway. I treat fish the same way as corals - I assume they have every bad thing known to man and (prophylactically) treat them as such.
Fwiw, you can't assume that the fish had anchor worms when you sold it. It could very well have picked those up at its new home.
I wouldn't blame you for it, maybe the buyer hasn't replied because he doesn't blame you for it either. Maybe he doesn't care.