Yehupetz.com

If a Jew walks into your store where you sell cutlery, and as you are counting the forks and knives he's buying, he tells you that they are for eating lobster, do you then have to refuse the sale?

Probably.

you can tell him that you're selling it to him but you only let him use it for kosher food.

It is mesaeya (helping someone do an aveirah) -- he can get it from someone else, but it's still not allowed.