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Greed is a negative (wicked) state of the heart (mind) were collecting money is not governed by moral standards especially with regard to fellow human beings (a key element is the employment of deception, for example). Also, a greedy person is not inclined to donate any of their money for those in need. In Luke 11:39 it is stated "And the Lord said unto him, Now do ye Pharisees make clean the outside of the cup and the platter; but your inward part is full of ravening and wickedness".
There is nothing in the bible that indicates collection hobbies are greed (as long as no moral standards with regard to collection dealings are violated).
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The fruit of the Spirit that counters greed is temperance. (Galatians 5 19-26).
Everything you do can be placed somewhere on a sliding scale between the sin and its complementary fruit of the spirit. Very rarely is amassing one item over another ipso facto greed.
If collection becomes obsessive, it might be time to step back and reflect on intentions. An obsession can lead to other sin such as envy for that one stamp you can't find but the store / a friend has.