Prior to your appointment of any auctioneer, you will be asked to sign an appointment letter which clearly stated the commission / fee amount you need to pay notwithstanding the sale is successful. Hence, you must consider concisely the marketability of your saleing property.
However, no one could provide guarantee to you that your property must be sold by any auctioneers or agents as it related to a lot of factors which include property quality, price, current market conditions, etc.
If you can not dispose of your property via agents for a considerable length of time. It is worthwhile to put for sale by public auction as an additional channel to faciliate its sale.
Do you understand that the auctioneer will incurred a lot expenses such as printing of poster & conditions of sale, open day manpower, rent for auction venue (most likely hotel function room) and clerical overhead, etc. Therefore, their operation costs are differ from those property agents.
By the way, auction sale may be an effective way to promote those properties which has been put for sale via agents but in vain.