To mark an item down or mark its price down means to reduce its price. A toy store has marked down the Sonic Hedgehog computer game Clothes are the best bargain, with many items marked down. = reduce mark up
If a teacher marks a student down, the teacher puts a lower grade on the student's work because of a mistake that has been made. If you mark each other's work, they don't mark you down because then you can mark them down
If you mark something down, you write it down. I tend to forget things unless I mark them down As he marks down the prices, he stops now and then to pack things into a large bag
If you mark someone down as a particular type of person, especially a type that you do not like, you consider that they have the qualities which make them that type of person. If he'd taken that five pounds, I would have marked him down as a greedy fool