It depends, a pricedrop is usually tied with the stock on the marketboards.
Unless indeed it's something one person wants to get rid of fast but those are usually snagged quick enough for anyone to notice unless they specifically look for it in the price history.
If I see a lot of stock on the boards I usually hold on to the items for a bit and try to sell them when the supply drops again.

If supply stays at a high point but the items are still worth selling I'll just repost them at the lower price and find another resource in the mean time.
Otherwise I'll just take my losses and npc the whole load.