~Marketplace Vending~ There are a few ways you can vend in the marketplace. The basic idea is to buy an item at a low price and sell it at a higher price so YOU make profit! You have to be fast though, otherwise someone might buy the lower priced item to make themselves profit! You should probably have a pretty fast browser and I suggest to save your password when you go and buy items. Low priced items that should be a higher price sell fast (like within a few seconds)!
Sometimes people make the item's price low because they want it to sell fast, some do it by total accident.
~ At first when you start looking for items to do this with, you may want to start with one item just to get the feel of it. When you know what item you want to start vending with, refresh your browser and try your luck. It is best at first to watch how the item is selling at at what prices it is selling for. After a while you can see just at what price you want to buy the item for and at what price you want to sell it at so you can make a profit. Remember, when you sell the item the MP takes 2% away from the sale. You may want the first item you start out with to be a cheaper item(unless you have enough gold). I don't suggest vending store items as most of them don't sell that fast. The best items (if you have about 50k in gold or more) are cash shop items that are in the store at the time like the Suspicious Package. You can also find some cheaper items that you can buy and sell quickly (like ink), you just have to look. At first it may take a while to learn and you may lose some gold, but after a while you will get the hang of it and it is quite fun! At higher gold prices you can make VERY good profits.
~Another way to vend (as shown in the picture below), is through watching the newest items put onto the MP. In this picture I found some ink that was WAY below the normal price. Ink stacks so you can buy and sell in multiples of the same color. The normal price of these inks at the time were 92 gold a piece. This person put them on the MP for WAY cheaper than that. That would be quite a good profit for someone just starting out. I underlined and pointed to where to go to get to this page. I suggest you know the fast selling items before you start doing it this way. But this way is also great if you want to know which items are selling fast and at what prices they are selling for.

~One more way... it is kind of like vending only a little different. You can sell regular shop items from your inventory for half the price that the store sells it at. Some people put store items way below half of the store price. If you buy that item and sell it from your inventory, you get profit!
Here is a pic of some carpet that is under the sell back price.

And here is where you sell it from your inventory, just click on the item you want to sell and this pops up.

Vending takes some work but it is definitely worth it. I personally have made around 400k with vending and that was only in a short period of time. Have patience and have fun!!