I'll give you
example with addon from Classic
Check the video under "Gold trade".
On the victim side, the frame is manipulated as well and it doesn't even show that you, as victim are offering all of your gold on the trade.
In short you're able to execute any lua code you want, even Logout() to instantly log out the victim after the trade has been finished.
With so many addons (especially backports of fancy classic/retail stuff) out there, and players installing literally everything without knowing what is inside makes them potential TAs of "I've logged and all of my gold is missing. HELP". Some intentionally injected, some just poorly filterered by the og author(s), but it's fact that nobody's reviewing the quality of any of these addons or at least nobody care to do it for the community's sake being pirate server and ungrateful work
So without addon with own frames and checks managing the trade window, telling you exactly what you're currently trading nothing will help. Default frames are/will be default for any client and it's cake to manipulate knowing them even if there is integrated await or something else