I know a potter who sells black and white crackle raku pieces that they have "fixed up" , coloring in sections with a black SHARPIE because the black parts weren't dark or even enough.
This person sells these pieces as seconds, but doesn't specify that the flaw has to do with using a sharpie to make the black sections blacker.
My own feeling is that this borders on a deceptive practice unless it is clearly disclosed. But the problem with this is, even if disclosed, the flaw is likely only disclosed to the first purchaser, not subsequent purchasers. Pottery can last a long time in the world.
Someone purchasing raku expects that the effects were produced by the raku firing, not by painting on a finish.
This is a little different from your case, but still, a repaired piece , out in the world, is an angry customer waiting to happen.