5buying it earlier than intended
The stock check is not strictly about what you have killed. Carry a mechanical boss summoning item over from a finished world and the Traveling Merchant will offer this in a world that has not even broken an altar yet. In a fresh yoyo run that is a large jump in power for nothing but gold.
where it sits
Shop weapons have the advantage that no farming is involved and the disadvantage that the merchant shows up when he feels like it. If you want it, keep the money spare and check him every visit. As a yoyo it covers the stretch between the early hardmode options and the ones locked behind Plantera.
the name
It is named after a real yoyo, the CODE2 made by One Drop, whose design came from Jason Rodriguez. The palette looks like it was cast from luminite at a glance, but the colors trace back to the physical toy rather than the ore.