A checker if PM has been read

[goodieboy]

Not too sure if this has been suggested before, but boy would this be useful. This way, we'd know if someone is offline or perhaps not too interested in your trade, and the buyer/seller would not be left wondering if the other guy has read the PM or not.

[j03]

This is already a feature, you can send something like a receipt to confirm if the receiver has read your message or not.

But this is really just a basic form of invasion of privacy. :P