This is exactly the point I was making - when someone buys a new thing the money circulates in the economy. They (sort of, in the case of Banksy) paid for it to be made. They created work. When it's just two rich people buying existing things from one another the money doesn't leave that loop, no additional work is done, and there's no real benefit to the economy.

To be honest I'm just making a pedantic point about Daniel's post where he said every purchase trickles down eventually. That's just not true in a small number of very high value purchases like art. If you've got buckets of cash it's better for the economy to buy a shiny new yacht instead of a painting. Nice problem to have I guess.