When you list something on the market, the sell form now shows the accepted price range under the trade box, for example "Accepted price for 500 iron: 12,500 to 150,000 coins". It updates as you change the item, the amount or the currency, so you know what counts as a fair trade before you post.

The "This is not a fair trade" message now names both sides too, for example "You must offer at least 10,000 iron for 750 credits", plus the accepted range.

Tip: check whether Coins or Credits is selected. One credit is worth thousands of coins, so a price meant in coins but sent in credits is refused.