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I found it hard to sell a complete lower for a decent price so I just bought parts and put together a new upper for it. So I would say uppers are easier to sell but don't get stuck with a lower and no upper to use with it.
Only four? Something's going to fall off... . As to the original question...I've found it easier to sell uppers as opposed to complete rifles. "But I could buy a Norc for half that..." fair enough...go buy it then...