In cases where someone wants to leave negative/neutral feedback, they might hesitate because they are worried that the other person will leave them negative/neutral feedback in retaliation. It might be useful to hide the feedback given on an individual trade until both participants have left their feedback in order to prevent this.

What if one person doesn't leave feedback?

If someone doesnt leave me feedback, i remind them about it and they usually then leave the feedback. i do agree with hiding feedback until both have left it though.

Maybe something like a daily email notification telling you that you forgot to leave feedback would be a good idea to go along with this.