3.25 auction house listing?

Will a public sell tab with stackable items automatically list on the auction house?

Or is there anything protecting you from accidentally clicking in a valuable item like "Doctor card" in low price public sell tab?

Normally you get the whisper spam, but if it auto lists then its gone to a preorder.

Nope. Currency Exchange Market is via Faustus the Fence and requires a gold fee. Think of it as a buy it now market interface for the bulk item exchange part of the Path of Exile trade site, but only for stackable items.

Thought the fee only was for buying stuff

It's an exchange; both parties are buying what the other is selling. Whether you see yourself as "buying" or "selling", your interaction with the Currency Exchange is paying a fee to put up a trade order.

If a similar order is already on the market, it will complete immediately; otherwise, the order will remain until someone else puts up a matching order, or you remove the trade order yourself.