>wouldn't he make more money if he took mine then?

Marketing is far more complex then you're giving it credit for. Take the Factorio game, they don't have sales ever so the best time to buy the game is now. This both keeps people that buy things on sale even if they don't like it from getting it, and keeps other people that may wait for a sale and forget about it from not buying it now.

The same is true for limited numbers. Some people may want it and put it in the cart, but never actually buy it because there is no strong binary motivator. This motivator can actually increase sales quickly and ensure you dont hold inventory for long periods of time.

Also things are commonly bought in batches to reduce price. Your one painting later could either be much more expensive or require the artist to buy 50/100 units at once that risk becoming stuck inventory.