In order to reduce the number of pointless trades, and to protect traders from accidental bad experiences, the trade office will do a check of the items you are attempting to trade, based on a rough value.

This means that if you are trying to trade 1 coin for a credit, the game will tell you that is not a fair trade.

However it will also protect you if you are accidentally trying to trade 100 credits for 10000 coins and will warn you to check your values.