1) New Deckbox (and many other similar site) members almost always send first. It's unfortunate that your trade partner didn't specify this in the trade, but it is the general practice. He should have made sure you were aware of this. In the end you both got the cards from the trade (ignoring condition and such for the moment) so no harm was really done here.
2) His cards arriving damaged. You say it was poor packaging and that could very well be the case. However, it sounds like the first thing you did was leave him negative feedback instead of trying to find a positive resolution.
3) His negative feedback in this case is totally justified. The entirety of his feedback relates to what happened after the cards were received.
4) Sebi is the only person that manages this site, so he is very busy.
You have two options:
1) Leave both negative feedbacks and both of you move on with your lives.
2, and the significantly better option) Work out a reasonable resolution that you are both satisfied with. The options for which are both of you returning the cards, him sending replacements, him purchasing replacements, or him sending cash through paypal (note that cash value does not equal trade value). Getting damaged cards sucks. Receiving negative feedback for something that you could have remedied sucks. Not being on Deckbox sucks. If you and your trade partner can work something out, Sebi can reset the feedback for the trade so you can both leave positive feedback when it gets resolved.