I'm a bit confused on your first suggestion now. Are you saying to only check for a single item when creating a chest shop? The way I read your suggestion was that chest shops would adapt to whatever item was inside of them.

If it's only in relation to creating shops, it's still got some other things you need to think about. With trade shops (not buy or sell, but trade shops), would the first item just be the default option for the first item in the trade? Would people still need to punch the shop sign with the second item in the trade, or would they put both items inside of the trade chest?

On top of that, the current system is something simple that players are familiar with, and helps keep it clear what you are making a shop for. Adding in this possible defaulting system opens the door for people to accidentally make shops just because they forgot they left a block in a chest. Or a friend could troll them mid process, throwing in a block before they make the shop, resulting in the shop sign automatically defaulting to that.

Users trying to setup their first buy shop could also run into issues. If someone placed a chest, but some grass inside of it, and then tried to make a buy shop using your default options, it'd cancel their shop creation and tell them that they can't trade grass. Yes, we could just check if the single block in there is grass, or another illegal to trade item, and ignore it, but that's even more unneeded code to add, as well as another layer of confusion for players to deal with when learning chest shops.