... Only the open source code gives us some level of confidence to trust it but normal user will definitely not appreciate it because not all knew how to read and examine code.
...
Unless I'm missing something here - the code doesn't matter that much in this case if at all. You can verify each transaction on blockchain, without the need of having any coding knowledge, and see all the auction bids. It's more about who controls the auction addresses and whether they'll follow their own rules if the game gets serious and the bids accumulate to a substantial amount.