No. You make an offer. If goods exist in that location at or below your offered price, you buy the goods. If no goods are at or below your price, you get an error message. If your order can be partially filled at that price, there's a checkbox you can turn on for 'Allow partial order', and if you have that turned on, you'll buy as many as are available at your price.

If multiple listings could fulfill your purchase, you get the listing made with the lowest asking price. If multiple listings are tied for asking price, you get the oldest listing.

You never see the actual asking price. If you pay 300d for an item, the system records the purchase as 300d, regardless of what the person might have listed for.

(In FFXI I made a habit of offering 1g for big-ticket items just in case someone thought they were being clever. You'd be surprised how many times I got something worth many tens of thousands of gil for 1g because people thought they could 'game' the system.)