New Items
New items are a no-no. You don't want to buy them when they're new, cause that's when they are the most expensive. Yeah, the first few people to get them make out a lot cause they essentially "set" the price, but as more and more people get this item, the value drops as people try to sell it off.
Eventually an equilibrium exists that sets a standard market price, but this is only after the price drops a lot. If you want to trade new items, make sure to buy and sell within a few hours. Don't "hold" the item, expecting the value to go up. It never does.