1) This typically a mistake. For example, if the price is 3,500 you will see players list for 350 / 35,000 / 350,000 etc. It looks though like they are only pricing them high as the 350 gold crate will only be ~listed~ for a second or two before it is bought, while the high ones just sit there.
2) Yes.
3) A larger trap, and the trap stays out for longer for mobs to trigger it.