I think a safe trade system would be cool and prevent scamming. /safetrade (name) would bring up a GUI menu in which you could input your items and agree when you are ready.
Oh I didn't know about the Redstone. Also for trade chestshops, people have to own the item first, this would be a way for people to get an item they didn’t already have
There are various ways to safe trade items, you should always record or screenshot your trades use trade shops if possible or have a mod do the trade.
In the occasion that I have to trade without a mod, I use a chest or shulker to trade. I place the items I'm trading in the chest and trust the person so that they can remove them. In this way, it is logged that the player removed items from the chest. If they do decide to leave without paying, the chestlog can reliably be used as evidence to recover the items lost
