What items do you see or have selling the most??

That's difficult to answer, because there are so many potential sales items:

- armor, weapons, mods, and consumables you can craft

- crafting materials you can collect

- cartel market goods, drops from Ops and FPs

What I can tell from my personal experience:

- most toons can only wield one weapon, rarely two. But all have six armor slots to cover, so there is always more demand for armor or clothing than for weapons

- mods for orange gear are in more or less constant demand, though greens don't sell as well as blues or purples

- ditto for consumables, with the same qualification about quality

In oder to turn a profit, no matter what you're selling, you must observe the market. Whether you have mats to sell, bind-on-equip loot, or are thinking of crafting an item:

first do a search on GTN to see how many other people are selling the same (or comparable) stuff, and what they charge for it.

If the market is crowded, wait for a day or two, then check again. If it is near empty, go for it.

When new desirable Cartel Market goods are introduced, in the first few days demand for them can go through the roof. One example: the Regal Miniskirt . Prices have not dropped below 75k AFAIK, most of the time they sell for 80-100k.