I left neutral feedback when a nice lady shipped me a Kawasaki Mach III crank in a cardboard fruit box. The box was promptly seized by Australian quarantine & destroyed. I was damned lucky they re-packaged the crank in another box & gave it to me (after a week & a half delay), but unlucky that the box was too big so the crank could roll around unhindered.. hopefully it will be okay. She should've known better 'cause if you go to the USPS website & look at the shipping rules for Australia, one of the top ones is "Don't use food boxes". She was damned lucky I didn't leave negative feedback, the only thing that saved her from that is the polite & apologetic e-mail she sent me.
You shouldn't be so hard with her. Each country has its own regulations when it comes to international mail, so you can't ask sellers to get informed about each one of them. For example, in Spain you don't have to fill any customs form regardless of which country your box goes to, so I guess many of them gets opened when they arrive. It would be easier if you prepare a few basic rules and send them to the seller. For example, I always ask the seller not to declare a value higher than 50 bucks, so I don't have to pay taxes in Spain, and I always ask them to specify that the parts are "used", that way the low price will make sense.
Anyway, when I recycle used boxes I use to wrap them with kraft paper. I don't want people thinking that what I send is what the box says and not crappy used bike parts. The spanish custom only opened one of my boxes once -I knew it for the tape-, and i suspect that it was because the seller uses an almost-new Yakima car rack box to send me a crappy used handlebar. They probably wanted to check if the seller was lying with the CN22 form...
Raul