The Market is present in all four villages and also within the hunter's hub. It's a place to buy items. Most of them you can gather or combine to obtain, but some of them are exclusive to the Market (Trap Tools, books, etc.). Every now and then, the Market will have a half-off sale . This is the time to splurge on whatever you need, be it Potions, Ammo, or anything else (books, however, will never go half-off, you have to buy them at full price. Printing is expensive!).
Some of the items at the Market can also be found by talking to Neko (means "Cat") at the Palico Ranch. If you find yourself too lazy to gather, drowning in zenny, or you need an emergency refill on supplies, feel free to stop by the Market at any time. It's a great place to buy your basic Normal2 and Normal3 ammo if you are a Gunner, especially in the early game when combining supplies are tight. Be sure to buy the Combo Books as they are available, as they increase your combination rate if they are sitting in your item box or your bag.
I recommend you purchase 2 copies of Book 1 and Book 2. As you work your way up the ranks, you'll find yourself needing to bring combination ingredients on quests to combine on the fly and limit break item carry caps. Combination books ensure that you won't get too much Garbage.