Sellers are allowed to set two thresholds: “reject any offer lower than” and “accept any offer higher than.” Sellers may use one, both, or neither of these options. However, some do use “accept any offer higher than” and set a price that is considerably lower than the Buy It Now price.
As a buyer, if you know that you would buy the item at the Buy It Now price, you should begin by making an offer. If the seller has set the “accept any offer higher than” option, you may immediately win the item with a lower bid. That is, the “auction” is complete at that point, and the item is yours.
What if this doesn’t happen? While you cannot submit a higher offer, you still have the option of buying the item at the Buy It Now price. So make your first bid count — don’t lowball.