Jujutsu Infinite items Common to Limited/Unobtainable, across Cursed Tools / Weapons, Armor / Sets, Domain Shards, Maximum Scrolls and Demon Fingers. Every seller is identity-checked, and every payment is held in escrow until you confirm the order arrived.
Through an in-game trade with the seller. The chat on the order page is recorded, so arrange the trade there.
Yes. The seller is only paid after you confirm you received the item.