Please help me with this donation system i want just to know a thing, not a full script?

Hello! My name is Hate_ZEU i`ve been scripting on roblox for almost 3 years now but i encountered something very weirdly. I saw a game donation inc where players can buy stuff from eachother inside the game. Of course i m a very advanced scripter but i ve never seen something like this. Can you tell me what event is exactly used to get all the player s items that he is selling ?