Definitely get nitro if you're planning on using an Angel. The Angel was designed with N2 in mind because Co2 can wreak havoc on your internals. Since N2/CA doesn't freeze, it doesn't have the possibility of wearing our your internals and causing you to buy them all over again (or just buy a new gun outright).
Why is your friend asking for only $50 for his Angel? Those markers, even models that are a few years old, go for at least $350 up to over $1000. Either your friend is really, really foolish in selling his so cheap or it's in really sad shape.
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Back from a two year LDS mission in the Bay Area.
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