We sold some of his Ninjago sets that they no longer make and he made enough to fund most of the cost of his first gaming PC. Some of them made 5 times what I paid for them! The Star Wars, Marvel and Castle stuff won't be getting sold. I regret selling all mine and he can make that choice when and if he has children of his own.
Sorting all this out has put me off lego for probably the rest of my life.
The amount of conversations I've had along the lines of:
Me "I can't find the winter soldiers left arm"
Him "It's on Princess Leia"
Me "So where's Leias arm?"
Him "It's on Mary Jane with venoms head"
Me "So where is MJ's arm"
Him "I dunno"