There a couple of ways to sell your unwanted items, merchant mode for P2P players, the Market or simply advertising in the correct channel. Putting things in the Market is just like putting your items on Ebay under a specific category but the problem is that it incurs a charge just by listing your items with the Market. Going into merchant mode, and under the restriction of only 6 merchants are allowed on a map (except for certain specific areas), only people passing by your location MIGHT take a look at it so therefore it's up to how you price your items so that the people who bothers to browse through your list will give a bit more thought to getting that item off you.
Due to the fact that Dofus doesn't have any passive spending (like bills, food and lodging), there is thus no urgency to acquire a steady stream of income.... which is why a merchant selling certain items will still be selling those items 1 week from now, or even 1 month from now. There are 2 ways to selling things; Sell cheap and it goes fast, or sell expensive and it goes slower but earns you a higher profit (Think 'fast' food compared to 'slow' restaurant food) so it's up to you how to strike a balance between the two ends.
Remember the Two Golden Rules:
1) An item is worth as much as someone is willing to pay for it.
2) An item is worth nothing if no one is going to buy it.