This particular note I just listed for sale is fun, what's called a "flipper". It's a 2021 (most current) Series with the serial number composed only of 0s, 6s and 9s such that it reads as a number upside down or right side up.
For only being the three numbers used it is also what's called a "trinary". Had that "0" been a "6" this note would double or even triple in value for it. Had it been a "9"? Wow... 10 or 20x.

https://www.ebay.com/itm/137418959578
See all my notes for sale:
https://www.ebay.com/usr/mpoloukhine

Banner hunting day yesterday. I go through about $1K brick a week scouring US $1 FRN and haven't seen one of these in the wild for probably a year.
Got TWO 2013B star notes within the duplicate range, both FW printed notes (the ones that were accidentally reprinted, not the original printing) in one $1K brick!

I'll be listing them on eBay in a day or two. If anyone here wants them, let me know and I'll get you the item number as soon as I list. Or make me an offer.

One of my recent sales.

The serial number contains eight consecutive digits (01234567) but all scattered. Pretty rare, these push into the higher value notes you can reasonably expect to find in the wild (like I did with this one)

I’ve got a few for sale along with lots of other types.

Another sale from my pile o cash. Not a usual find in the wild, but there it was :-)

https://www.ebay.com/itm/135777430750

"True" trinary note. The serial number contains only three different digits (trinary) and in this case, 0, 1 & 2 ("true" ... as the next step up from a binary 1s & 0s note). This one also very low sum (add up all the digits and you get 5, the lowest possible for a trinary being 3).

Found this one in the wild, selling, excellent condition.
https://www.ebay.com/itm/136962912485

See my eBay page for a bunch of additional new listings.
https://www.ebay.com/usr/mpoloukhine