Hello, I propose this idea because I find that it is quite disabling that it is not possible...
It would be nice, and more logical in my opinion, to be able to send messages with an exchange, because when one is in progress, it monopolizes the messaging, and it becomes impossible to talk with the other person, as long as the exchange is not finished. This can create problems when proposing offers for example, or simply, I find that it would be more polite to be able to send a word in the exchange...
As it stands, I find it a little unpleasant to end up sending trades without a message, and that in addition, it removes any possibility of discussion. It's a bit scary to come across as the rude person, simply because you can't send even a quick message to thank the other person, or to ask if the exchange would be ok with them...