Diablo 3: the way that Bidding Work

For equipment such as armor and weapons, players can set a maximum bid to join automatic bidding; the system will place a bid at the lowest possible winning price automatically, in 5% increments, up to the maximum the player sets.
If you want to buy a diablo 3 item immediately, you can also choose to buy out the auction for the listed buyout price provided that the seller offers one. Players can check the status of their bids on the "Auctions" tab of the auction house interface.
Our goal with the commodities system is to estimate “search spam” for many commonly auctioned items, making it so players need not to flip through dozens of pages of listings to ensure they’re getting the best value.
For commodities like gems, crafting materials, gold, and other stackable items, players can just search for the commodity they want to buy and enter the quantity they would like to buy. One thing: players are not allowed to bid on commodities, but can only buy them out.
The auction house system will show the player with an estimated cost automatically according to the best prices it was able to find on the market. The player can then choose to proceed with the transaction, at which point the auction house system will re-scan the marketplace for new pricing. If the updated price is lower than or just the same with the quoted price, the transaction will go through at the updated price. And if the updated price is higher than the estimated price, the transaction is not able to go through, and the player can choose to search again.
Source: battle. net.