Remainder marks look like defects to new shoppers but are intentional. They exist to stop discounted overstock from being returned to the publisher for full credit, which is why they are placed where they cannot be trimmed away without visibly shortening the book.
Inspect the closed page edges before you buy. A marker dot is minor; a heavy sprayed band across the bottom edge is more noticeable and affects perceived condition more. The interior and binding are otherwise brand new in most cases.
For pricing, remainder marks generally push a copy out of 'New' and into 'Like New' or 'Very Good' territory on most marketplaces, and the mark must be disclosed. Expect a meaningful discount versus an unmarked copy of the same title.