Initially, after stepping on one in the middle of the night, any that I found left on the floor went right into the trash. I assume he dug them back out, being a kid and all. However, he learned.
Eventually, he amassed a giant collection. With his permission, I gave them away to a kid that lives nearby. There were six large tote boxes and several dozen box sets. That kid now wants to be an engineer, though my son went into biology.
So, they are really great and it's even better that the old blocks still work with the new blocks. Just look out, they're caltrops, on par with a four-sided die.
Yeah, watch out for them when you're not wearing shoes. Evil buggers.
https://shop.lego.com/en-US/Pick-a-Brick
Order all of them.
If I'd decided to sell out of my collection instead of giving it away, I'd have had some parallel cases - any large enough collection is liable to include a small but significant proportion of one-off parts. I'm not sure a 2x4 black brick is likely enough to sell that it'd be worth listing alone, but I suppose someone with a sufficiently completist bent might go to the trouble.