I have noticed that some non-Jewish owned produce stores in U.S. have someone trimming the rotten or ugly leaves off the edge of lettuce, cabbage and other similar leafy vegetables. The knife used is cold but it comes from an unknown source. The same knife may have been used to cut unkosher meat, fish, onions, etc. There is no knowing where the knife has been used.
Can someone buy such produce? Would it be sufficient to manually remove an extra layer of leaves from these vegetables after buying them?