This case continues to fascinate - due to there being no resolution. Wanted to expand on a post I read on here recently, but unfortunately can't find now. I.e., another possibility that hasn't been discussed that much is this:
D.B. Cooper died in the process of landing due to injury/drowning, etc. And, his body/parachute/bag/rest of the money (that wasn't found on the river-bank much later in 1980) was found with him at some point soon after. The person/persons that found him realized who he was due to the high-profile nature of the case/news reports/etc. - i.e., I suspect that even in 1971, this made big headlines. So, the person/persons decided to dispose of the body/parachute/bag (somewhere where they would never be found) & keep the money. They weren't greedy, so just spent this a little at a time, making small purchases at numerous places over the years. And, they never deposited any of the money in a bank. That's why this was never traced back to them. Note this person/persons may (or may not) have been local to the area where Cooper jumped. But, if they were - they could have easily moved right afterwards.
Yes, this is far-fetched. But, it would explain why his body/parachute/bag/etc. was never found.
Going along with this, if this had happened & the person/persons used the Cooper cash to buy groceries/gas/etc. - I don't see how every store/business in the U.S. could have conceivably tracked the money using the serial #'s, etc. IMHO, this would have been impossible in 1971. And, it would probably be impossible even today.