investments and having fun building a set are two different things.
investors invest in items they believe will go up in value. collectors of a set collect every card in the set, but rarely does a set sell as a set, they usually are broken up and sold piecemeal. so it ends up being each individual card as investments, but each card in a set is not the same as far as demand and price increases (see '52 mantle).
investors would buy a rock today if they believed they could sell it for double tomorrow, or next year. but if you search long and hard for that common name card in real nice condition to complete your set, and you sell it when the set is broken up and it didnt command much more than you bought it for, it's not an investment, its a hobby.
if you are collecting for fun in a hobby, don't look at it from an investment standpoint, it's fun.
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