Because of these situations, it's important to have your Indiegala account linked to your Steam account. You could have contacted the user via Steam, explained the situation, and resolved the problem without needing to leave negative feedback. To correct the feedback, just send a message to...
1st - Link your steam account to your indiegala account. People need to know who they are going to make the trade with.
2nd - I unlocked your Steam profile, so people can see your library and wish list. To make offers for games that interest you or that you at least don't have.
This also...
It's been more than six months and this problem hasn't been fixed... I believe that the trade section was intentionally abandoned, that is, deactivated. What a shame, it was a very useful tool for exchanging digital games.