I don't know about a specific reddit post, but it's a generally accepted fact that a lot of the trade in fine art is used to accomplish a lot of money laundering.
I imagine you saw it in a thread about NFTs? That's where it's been topical lately.
Googling "fine art money laundering" or "nft money laundering" will bring up a lot of articles of you're interested in the how and why.
A very basic and simple example might be that you have a bunch of drug money to launder.
Buy one Art for £10k of "clean" money.
Sell it at auction.
Bid anonymously through a proxy and buy it from yourself for as much as you reasonable can, let's say £50k.
Pay with the drug money.
Now when the authorities ask you where you got that £50k you have a very public paper trail.