I don't agree with wam03 at all. Carefully defining terms to apportion blame away from yourself would be very Clintonesque. The 50-50 idea is ridiculous.
I was in this very situation a few months back...except I was the one who did not receive the knife. This was an AR for GB trade. I sent the GB priority mail...insured...with delivery confirmation. It got to it's recipient within 3 days. The other party tossed the knife to be shipped to me in a big envelope with some plastic wrapped around it...slapped a few stamps on it...and dropped it in the mailbox. Pretty dumb, if you ask me.
Anyway...two weeks later...still no knife on my end. I contacted him and he'd already traded the GB. Swell. My feeling was that he was gonna be on the hook for the $$$ 'cause he was not "with it" enough to insure the knife. In addition, without any proof of his mailing it...there was no proof that a knife had ever been sent to me. In essence, he was at my mercy. I could have been lying...said the AR never arrived...what was he gonna do?
Well...to make a long story short...and to give you some hope...it eventually showed up. I want to say after about four weeks...but my memory is a bit fuzzy. It was fine...and he had indeed mailed it when he said he had. BUT...he almost ate the money. It's an accident waiting to happen. You put yourself at risk from thieves in the post office...and unscrupulous buyers...just to save a few bucks.
I also had a Ferret I bought off the board here show up after three weeks...in its box...which was in a bag (considerately provided by the USPS)....with about half a cup of water in said bag. It was soaked. I was about to cry...but...the seller had the knife padded...and in a ziploc bag. It was none the worse for wear.
Moral of the story is...ship knives in baggies

and use delivery confirmation...and insurance if it's an expensive knife. The other moral is that sometimes...the stupid USPS takes a long time to get things from place to place...but they do arrive.
Keep a good thought. It may yet get there...