I think we just don't know where our items come from unless we're the original first owner so it's hard to tell if an item stolen or not. I'm going to mention a couple of tips to you in case your concerned that you're purchasing a stolen item:
Firstly, it wouldn't hurt to Contact Cipsoft and submit a ticket advising them you're considering purchasing an item that a player is selling. Tell them the seller's name and world as well as yours, what the item is, and how you want to know if your okay to buy the item as you heard rumors that players can get deleted for owning an item that's stolen.
Do your research unless there's solid proof like the item being in their house for years or other sources out there indicating player x owns a winged helmet from this contest back is 2020.
Be careful buying from level 8's, isn't it weird how someone is selling a rare as a level 8... I never like that it seems like their scared about who they are or probably just visiting from another world.
Any item can be stolen, how we handle the situation can help protect us and our accounts. I'm sure everything varies on a case by case basis as to if someone gets deleted or not due to owning a stolen item, I've never experienced this myself. It doesn't seem like you can get deleted for buying a Serpent Sword from someone that was stolen. I think this situation could probably only occur in a very rare circumstance like shown in the related question: https://www.tibiaqa.com/15226/is-it-against-the-rules-to-steal-another-players-item-obtain-stolen-item?show=15233#a15233