I found a vintage LEGO set from the early 1980s, with box. It is missing 25% of the pieces. If I purchase the rest of the pieces separately, is it acceptable practice to sell that as a "complete set"?
New bricks of the same kind often comes from a slightly different mold. And pieces from different year will look different, especially white ones. So there is a high chance that difference will be seen.
Most ethical thing to do is to sell it as complete set, but in description clearly state what bricks was replaced.