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The 5Star was obtainable by reaching level 45 on the police team in OG Season 1. Alongside that you needed to buy it for 10k cash.
5Star already has an insanely low copy count of around ~4.1K, paired with its insanely low MU count (currently around ~80), it's no surprise that its an extremely rare item to spot in trading. It's similar in rarity compared to other extremely rare items, such as Wagonwheel, Iron Rock or Center Lock.
Supply is widely distributed — no single holder owns enough to register as a hoarder.