This is the official response from the FAQ thread
Q: How do I know if a trade offer/message was read?
A: Right now you can’t. This could be considered a privacy issue as some users do not want others to know if they have read their mail or not. We do not consider this a bug as it was intentionally not put into place although we may reconsider this in the future.
Get real. Having a flag to indicate whether a trade offer has been read is not violating anyone's privacy. You are not reading anyone's private thoughts or emails. It is no different from a read receipt request when sending an email from outlook. We deserve better than a lame excuse like this for not fixing something that needs fixing.
How about this. Give us the flag. If the person receiving the trade offer does not want sender to know they have read the offer then they just don't read the offer. If they value privacy that much they should respect the privacy of the trade offer sender and not read the communication.
Try running a poll. I guarantee those in favor of having such a flag would dominate the poll. Unfortunately the improved boards do not have a poll function as the old boards did. So cannot actually prove this.
But how about a somewhat unscientific poll. How about everyone post here a one line either in support or opposition to having a flag to indicate whether a trade offer has been read?
And if there is by chance anyone who does want to keep hidden whether they have read a trade offer please give your reasons why you would want to keep the sender of the trade offer in the dark.