For the bagged kits there's no reason to open to inventory before selling, so if someone is saying that they did that they are probably just covering for the fact that the kit was already open when they got it, or they opened it and decided to sell instead of keep/build.

For boxed kits I think it's a good idea to buy the opened box since you can see what's really in it. I read on one of the forums sometime back that some guy bought either an Estes Sat V or an Estes Mercury Atlas sealed in shrink wrap and after more than a year had passed he opened it and found that it was missing a number of important components.

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I plan ahead that way I don't have to do anything right now.

Oh by the way, I'm not here just for the "olde" rocket discussions.