Publicly bashing the owner of a site isn't a great way to convince him to unsuspend you.
HOWEVER, according to the details you have provided it sounds like Sebi did make, in my opinion, a bad decision. The relevant Deckbox rule here is:
2.2. Sending cards. The sender is responsible for getting the cards to the destination. He will be held accountable for items lost in the mail if a form of tracking was not used; proof of sending is not sufficient.
If you as a sender do not have proof of delivery, resend your part of the deal, make monetary restitution or return the cards you received. Resending cards should always use a form of tracking, to avoid further disputes.
I believe the underlined sentence is the most important here. The other trader has no proof of delivery, whereas you do.
Now, for Sebi's viewpoint. He probably looked at your tracking info and saw that your cards were delivered. Then he looked at the other guy's tracking info and saw that nothing was delivered. My guess is that he's thinking that the USPS lost the package (at no fault to either trader ofc) so instead of 1 guy eating the entire loss, the loss will be split between the two traders. However, this is not the way that this should be handled according to the rule cited above.
Facts (According to what you've told us):
You sent your cards and have proof of delivery.
The other trader has no proof of either sending or delivery.
The rule states that if you have proof of delivery that you are protected.
The rule also states that if you DON'T have proof of delivery that you have to make restitution (by returning the cards or w/e)
Now, having spoken directly with Sebi myself numerous times and I payed close attention back when the Bad Trading Report forum was a public thing I can say with confidence that I've never seen him ban or suspend someone for no reason. I'm going to take a wild guess here and say that the language you used during this dispute was less than friendly at times. Directly threatening your trade partner with a mail fraud claim certainly didn't help the situation (even though it seems like you would actually be justified in doing so).
Also, Sebi has nothing to gain monetarily by suspending someone and he doesn't care about the cards themselves. He doesn't gain anything by you two trading. He also doesn't know either of you personally so he's not going to choose to screw one of you over for no reason.