Winning the 1000 Bitcoin Puzzle

I'm assuming most people are familiar with the 1000 BTC Challenge, but for context I'll paste the websites description:

"In 2015, in order to show the hugeness of the private key space (or maybe just for fun), someone created a "puzzle" where he chose keys in a certain smaller space and sent increasing amounts to each of those keys.

On 2017-07-11: funds from addresses #161—256 were moved to the same number of addresses of the lower range – thus increasing the amount of funds on them.

On 2019-05-31: the creator of the "puzzles" creates outgoing transactions with the value of 1000 satoshi for addresses #65, #70, #75, #80, #85, #90, #95, #100, #105, #110, #115, #120, #125, #130, #135, #140, #145, #150, #155, #160 with the aim of probably comparing the difficulty of finding a private key for the address from which such a transaction was carried out, and one that there is no transaction.

On 2023-04-16: somebody (maybe the owner) increased the unsolved puzzles prizes again by x10. Now the puzzle #66 prize is 6.6 BTC, #67 is 6.7 BTC and so on... puzzle #160 prize is 16 BTC.

1-65 and #70, #75, #80, #85, #90, #95, #100, #105, #110, #115, #120, #125 private keys have already been cracked.

Basically my question is, let's say someone finds the private key for one of the wallets and transfers it to their own. Eventually you'll want to cash out, how does this work with taxes? Do you just assume 50% is subject to capital gains with a $0 cost basis? Genuinely curious what people would think. Also would sending it to an exchange raise eyebrows or get you flagged? I just came across the puzzle and am running all these unnecessary hypotheticals out