"Isn't the only reason people buy baseball cards as collectables is that they think they will go up?"
1) No. Some people just like to have them because they're fans. The name "collectable" as opposed to "sellable" is a bit of a clue.
2) in any case, the test is not whether the value of the thing will go up, but whether the buyer reasonably believed the value would go up as part of a common enterprise with the seller. Sotheby's will never promise that your Van Gogh will go up in value, and if it did, it's not because Sotheby's was going to be doing anything about it.
Biting the hand that feeds IT
