Black Mesa Pin is a High Grade-rarity pin. Rarity sets the base drop odds, though what you actually pay also depends on market supply and demand.
Black Mesa Pin drops from Half-Life: Alyx Collectible Pins Capsule. Drops are random, so buying from a marketplace is usually faster and more predictable.
No. Black Mesa Pin is purely cosmetic — it does not change weapon performance, damage, accuracy, or any gameplay mechanic.