Is it ok though to offer trade never intending to do it, as a way to gauge what other player is willing to give out for a said card? And then backtrack after other player agrees to that trade?
Is it ok, if you don't have wheat at all, to start trading like "I would like to give out wheat in exchange for X, not having wheat at all in order to find out what other players are willing to give you for wheat? Is that allowed?
I don't think that there are any rules against it (Catan doesn't have much to say about when trade agreements become binding), but I would consider it poor sportsmanship lie about your trading plans in order to gain information and mislead your opponents. If you repeatedly act like this there's a reasonable chance your game group will sour on playing Catan with you.
While the rules don't explicitly outlaw offering of an illegal trade, the goal of behavior seems to be circumventing the spirit of the following rule: Keep your development cards hidden (in your hand) until you use them, so your opponents can't anticipate your play.
I would highly suggest that groups consider such behavior illegal, since allowing such a phishing expedition could slow down the game. Since the rules don't cover the situation, there is no official penalty. If the group determines the need for a house rule to penalize this behavior, I would suggest the following: If Player A accepts a trade offer from Player B but Player B does not have the resource available, Player A is entitled to make the trade the very next time Player B receives the resource(s) offered.
