I'm lucky i sold my whip a couple days ago, im quite good with luck like that, i usually sell items just before they drop a lot, i'm happy with whips dropping as i'll never have 85 slayer and i could always do with a whip at lower prices. My idea for why whips are dropping is the amount in circulation, more people getting 85 slayer = more whips, also it could do with even a few people selling say 40 whips each at a lower price, then as most people around them want to sell for a competative price they drop theirs, then a sort of chain reaction occurs and prices drop for a couple days, they might go back up but im not an expert at merchanting so i'm not sure.