Explain
Trading in Catan in terms of a car boot sale
Trading in Catan is just a miserable Sunday morning at a car boot sale where everyone is wearing a cagoule and nobody has any change.
On your turn, you are the only one allowed to haggle, clutching your hand of brick, lumber, wool, grain and ore like a box of chipped Minton teacups you found in the loft. You initiate the trade, offering a useless sheep to anyone who will listen, while the other players stand there with their arms folded, pretending they cannot hear you over the imaginary sound of a distant ice cream van. If nobody bites, you are forced to deal with the bank, which is essentially a grumpy man in a transit van who insists on a four-to-one exchange rate just to take your tat off his hands.
To get a better deal, you need to build a settlement on a harbour, which is the equivalent of paying the site organiser for a premium pitch near the entrance. Once you have that maritime presence, the bank drops to three-to-one, or even two-to-one for specific resources, much like how the guy in the van suddenly gets very interested in your rusty lawnmower if it is actually worth something. It is a cutthroat economy where the only currency is sheep and your dignity, and you will inevitably leave with less than you started with.
Where the analogy breaks downIn a real car boot sale, the bank cannot force you to give them four identical items for one of your choosing.