I think this is a tough question to answer because I guess it all depends on the items. The rarity, game world, worth and use is probably main factors. I've personally never looted an item before anyone else has but I know if I were to do that I would definitely start the price up high lol. I'd look at similar items and see how their priced, however I would increase it of course because it's a new item and everyone wants new items. It all depends on how much someone is willing to spend. I mean if you look at Cobra and Falcon items they started the price up so high I've never seen equipment cost so much in my years of playing Tibia. But it's because people knew how much they were worth and how much people will spend. A good tip would be to probably go in Trade Boards and let everyone know that you have an item that no one else has. Describe the item and sell that baby. I guess I want to touch on the main factors I think people look for in items:
Rarity would be how hard it took to loot it and how much supplies did you waste? Would you risk not selling it before everyone else had a chance to loot it?
Game world has a major play in this as an unpopulated server probably won't showcase a lot of buyers for example. But people will travel for a new hot item so it can be worth your while. Also the whole optional pvp and pvp type of ordeals where if there's already cheap rares then yeah you'll get less money.
Worth would be how much it means to you and the buyer. Will it make you or them happy?
Use like what is it? If it's new equipment then everyone would eventually get it and this will be a hot item for a while so start the price off right. If it's decoration that's another hot item as everyone loves new decor.