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 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
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 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