Placing a Bid will allow you to set how much you'd like to pay for an item. If you have the highest bid price for an item on the site, it will be up to a Seller to determine if they are interested in meeting you at your offer.

Buy Now immediately purchases your item from the Seller with the lowest Ask—if you like the current price of an item with respect to the size you’re trying to purchase, this is the easiest way to lock in with a Seller and make sure it doesn't slip through your fingers.