If auctioneers offered guarantees, bidding would increase and bids would go higher. Guarantees are a way for buyers to know what they are buying is a sound investment. Guarantees don't create a conflict of interest, they create a better system for everyone involved. Buyers and sellers can know a third party will evaluate what is true and what is not when it comes to an auction.
Yes, auctioneers should offer guarantees, because that gives seller the confidence to place their work or their pieces up for auction. Without a guarantee, some sellers might be hesitant to place their work for auction, not knowing what the outcome will be. A professional auctioneer can make a guarantee in order to entice additional business and earn a profit.