A collection is like taking care of a Bonsai Tree. It grows and you trim it, it grows and you trim it again. You are always looking for the perfect shape, and that shape changes over time.
That is how you need to think of your collection. What is the goal you are shooting for? Will a new purchase get you closer to that goal? What do you need to sell in your collection to get that new piece?
Recently I sold almost every piece of a certain aspect of my collection. It just wasn't growing the way I wanted, and the key pieces probably weren't going to surface or were too expensive. So I decided to do away with that "branch" of my collection.
Think about your collection, and see if you are growing it with a goal in mind.
And remember, it will never be "perfect".