Last year I bought an item off BoB, seller sent me my tracking number, when tracking on the website it continually showed "at CapeMail", after 5 days of this I phoned 0860 111 502 to inquire where it was to be told it had bn sent to the Grand Central Post Office by mistake but was bn held there waiting for me to collect it.
Next day when I went to pick it up I was informed it had bn collected earlier in the day...........by a gentleman with the surname of Salie.
After vigorously flashing my ID card around, asking if I looked anything like the gentleman in question the manager appeared to ask me to leave my details and he will get back to me in the course of the day.
No damn way, said I, if need be I will stay here all day progressively making more & more noise (I should mention I do have quite a loud voice) until either the idiot who imagines my name to be what it is not is found....or the elusive Mr.Salie is tracked down or I am given my parcel & will then be on my way. 20 minutes later my parcel miraculously appeared......apparently it had dropped behind a radiator or some such place.

My point is when you are given a tracking number that is your proof that it was posted, and registered is even better because you have the slip, the onus then falls on the buyer to collect the parcel which is why when sending a tracking number I include the telephone number & the website for track & trace details so that it can be tracked all along it's journey.
If your buyer were to make enough noise I am sure the parcel would be "found".