I'm not against this idea at all, I think it would be great to include higher amounts in the trade menu especially if that's how the economy is especially late on into the seasons, but /trade isn't the only way to do deals.
There's /ah and /pay - and while yeah the argument exists that they aren't the safest options, it's not like scamming is allowed. In the event you didn't get the money or the cubits all you'd really have to do is take a screenshot of the deal and of you sending the agreed funds, or contact a Moderator and have them look into it. I'm bringing this up just as the way you suggested of having to add $1m around 1,000 times is just impractical, and that there are other ways around this in the mean-time.
That being said! I do fully agree with higher amounts being added to the trade menu for peace of mind.