I have never bought ducks at an auction but I have bought other livestock. Call them and make sure that each grouping is housed separately (different stall/cage/or other containment) as to avoid mixing up birds or cross contaminating between owners as much as possible. The ones that don't tend to use a bait and switch method from what I have seen (you bid and pay for one you like and they give you a sick one). Also, make sure that you are either able to inspect the birds yourself or they have a letter of health from a vet. If you have any now at your place you will want to quarantine the new ones for safety. Either way you want to check their health as soon as you get them and address any problems. Good luck and happy bidding!