The Trading Post is arguably the most important building in the game, because if you do not correctly balance your trades, you'll either not acquire the items you need, or never accumulate the gold that you need for building upgrades because you are buying too much stuff.
Purchasing Items at the Trading Post
There are certain items that you will always need, and always be requesting, these depend on what difficulty level you are currently playing, but basically the essential items are those which the hunters need to be able to continue hunting.
So always make sure you have enough of the items that you need to craft the items sold in the Inn, Restaurant and Infirmary. If you cannot get these items then your hunters can't hunt and every stops.Tip:
Remember in an emergency you can also use gems, elemental and precious stones to craft certain items if you miss the regular materials.
Other Items are still important and you should purchase them, but just be aware to cut back on these purchases if you are trying to accumulate gold (Tips for more Gold