Comment by tuna-piano

Again, I'm not very knowledgable, but I think the first sale doctrine just says it's now your lego and you can sell the lego.

If you start copying the piece assortments ("lego recipes") that lego also sells, that's where I'm unsure that you're in a safe legal spot. Lego put time and effort making sets for specific purposes, and I would think that those "lego recipes" are possibly protected.

Especially if you were thinking of selling a large number of different sets, which would mean you'd be creating kind of a "lego recipe" catalog?

https://www.copyright.gov/fls/fl122.html