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.