Of course - there's no way to control what the sellers do, in the same way that there's no way to stop them from putting a $1 million price tag on some 1999 Sheng cake in their shop.
I'm sure you know this already, but there's no harm in bringing up the topic...
Value is all relative, and monetary value is defined by how much someone is prepared to pay for something. If consumers are willing to pay some price for something, then that item is worth that price to them. Someone starving to death would gladly hand over a fistful of banknotes for a nutritious meal, but others might only be prepared to pay a small sum.
For the people who hold these cakes that are in short supply, the choice to sell or not to sell is theirs. The price they would be willing to part with these cakes for is also theirs. With a commodity that is not easily replaceable they have to weigh up what that cake is worth to them and what a certain amount of money could buy for them in return.
I heard of someone in Taipei who was offered 5 times the market rate for one of his old cakes, but he turned it down. He had enough money, he didn't need more, but that cake was special to him - he couldn't buy another. That cake was worth more to him than 5 times the normal market rate.
I agree though - it's likely that there could be some rigging going on by some people. It doesn't matter to me because these cakes aren't worth that price to me. They may be to someone else though but that's a decision each of us has to make for ourselves.