Pannacotta - I thought someone might ask that!
First trick is not to choose new or used from the pull down menu when listing just selcet the '-' instead to leave it blank.
Be careful with your wording - I just tend to not put any reference to condition whatsoever.
Invite questions about the item in a prominent place on your listing - I always get people asking the condition of the nappies.
Put a sensible buy it now option so that they can be purchased before being discovered (that one works a treat at the moment and I have sold nappies in under an hour several times and got a price I was happy with).
You can either email bidders who haven't asked about the nappy's condition to make sure they are happy or let it run to the end and if the winning bidder hasn't asked let them know about the condition and give them the opportunity to back out of the sale without any issues.
Best not answer any questions from ebay nappy traders.
Of course you might get caught out but it is a chance you take. I don't rely on ebay now for my nappy sales but I have noticed a lot get through second hand these days.
btw saw the same nappies on ebay being sold as new which were also listed on usednappies as having been well used. Now that really pXXXXXXs me off....