You will want to check out Data Store Service: Data Stores | Roblox Creator Documentation. You can use the Item Name as your key and set it to be true using :SetAsync when it is purchased. You can use :GetAsync to see whether it has been purchased. This will require an intermediate level of scripting knowledge. From the code segment above it seems like you might be a beginner. I would highly recommend watching tutorials from AlvinBlox - YouTube or TheDevKing - YouTube. I hope this helps!
game:GetService("DataStoreService"):SetAsync("ServerData",{
NaturalRinnegan = true
}, "1") -- one for scope bascially data version
local data = game:GetService("DataStoreService"):GetAsync("ServerData")
if data.NaturalRinnegan == true then
warn("NATURAL RINNEGAN TAKEN")
end
-- you could alternatively use MemoryService.