Actually you can because horses don't take durability damage, only enchanted items/items with durability taken away aren't allowed in auction. This promo auction is perfectly fine This is my 12,000th post on the forums... woo!
You can refer to this ruling by a moderator here: https://empireminecraft.com/threads/auction-1-used-dancer-and-1-vault-voucher.53510/#post-961871 which shows auctions for used dancers are valid.
aaannnnddd the auction is now over! the preview sign on my res will be changed to an access sign as soon as the payment is received
