It would be interesting to see if they breed. Even though they bred in his tank, as he said, just be aware he might've had the right water chemistry. If your water doesn't match, they may not breed for you. It's a good deal on them either way.
they can breed, problem is they don't look after their eggs so tank mates (including other ghostknifes) will eat the eggs. This is why it is hard to tell if they bred.
You will see the females get really fat when breeding time. water change does help.