Of course you (as those in charge of the trade) are obliged to muster up the best trade you can. If you make unreasonable offers though, the rival club is going to tell you to get stuffed and you miss your big fish.
Remember when Essendon offered pick 35 to the Bulldogs to Scott West? It's why nobody ever deals with Essendon, because they're a pack of ....s at the trade table.
So by all means play hard ball, but you can't go too far the other way because the other club is going to slam the door on any possible trades.