Bidding In Ventari's Corner

We do not have any rule regarding this, the seller sets his own premises for the auction. If he neglects to mention that he does not accept z-keys as per your example, the bids should still be seen as valid, since neither you or the other bidder knew that the seller was not going to accept z-keys when you placed your bids.
Though z-keys a volatile currency at present, the other bidder might have converted the z-keys to ecto (at whatever rate he could) or might have offered to trade a like number of ecto instead of the z-keys if he had been the winner of the auction.

You thought 120 ecto was a fair deal when you placed the bid and that should not have changed because of the information you received later.
Consider also that the seller might well have opted not to sell to you at all if your bid had been lower (as is his right, whether stated or not). There was ample time for the Seller to make a mention that only ectos are acceptable. My bid would have stop at 105 Ectos as the remaining bids after that were in Z Keys. It is hard for a bidder to quantify which bid is higher or lower say for example, 105 Ectos against 110 Z Keys. So against 110 Z Keys, what number of Ectos should be bidded if 105 Ectos is considered lower? Who decides, if Seller keeps mum and makes no effort to clarify?

So if Seller had during the process of bidding say he does not want Z Keys, the bid would have ended at 105 Ectos instead of 120 Ectos. It is not a matter of paying the additional 15 Ectos but whether this is seen as being ethical.