I had a volcanic island come to me in a bubble mailer and a penny sleeve, as you can probably guess a corner was slightly bent but no permanent damage, i immediately contacted the seller and went through the whole "hey, for future reference, PLEASE USE A TOP LOADER etc etc" bit
Heres what you do:
1) TAKE PICTURES
2) contact the seller
3) try to work with the seller, if you want to keep the banged up cards, try and see if you guys can work something out where he can send whatever to make up the lost value
4) if you dont want to keep the cards, get a bubbler with tracking, put the cards into a toploader and send em back
5) cancel the trade, start a new trade if you really want to
6) inform the seller that top loaders are cheap and the preferred way to keep cards safe in transit
The best part about top loaders: you can reuse them! i have a bunch of the little buggers just sitting around from purchases/trades, easy to accumulate but hard as hell to get rid of
And if you REALLY want to make sure your cards get there straight, you can get one of those hard plastic cases, its more expensive but when youre sending out a large value of cardboard its worth it.