The Mat and Lego obstacles can be very useful to stay together for the off season. Addition tournaments, off season practice, and summer workshops are very helpful for getting a team ready to compete. I would recommend keeping it as a set. You could donate it to your school/ organization, sell it to another start-up team, or keep it for your own future teams, but keep it together.
There are a bunch of pieces, usually regular blocks that you won't really need again. I think the kids find it exciting to get some of them, and it won't hurt your future teams to let them have those pieces. I would definitely save any bars, gears or technic pieces for future teams. This year's spinner and strength machine are gold mines for those pieces!
I guess it really depends on if you will be coaching again. If you are, it is a team robot- not yours, but an FLL bot used by any team members of the future. If you're not, then it's not as easy a decision, but you said you were, so that's that with the machine.
Good luck fielding any questions from parents who believe they purchased a NXT machine ;)
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