You must type a command (/confirmtrade) to complete a trade after clicking a trade sign that costs more than 1k Reason: So people dont lose a lot of money if they accidently click on a trade sign.
While I know this frustration, as I once bought 10k worth of redstone lamps from the spawn buildshop instead of 1 noteblock (yes feel free to laugh) I don't think adding a command is worth it, I would just say be careful with what you click on and where.
Hmmmm sounds interesting but the best thing to do would just be to read the trade sign before you click on it. Infact make sure you read it twice just to be safe