If the condo residents all use a community garbage dumpster I would make regular vists to that dumpster to see if the shipping box were there (if it were shipped in a box). If it is, and if the package was in fact adressed incorrectly, the label on the package may show that. At the very least, if you find the box you will know for certain that the package was in fact delivered and that someone stole it.
If you can find the packaging, I would put up notices around the condo with big bold letters that say "STOLEN PROPERTY", a title like that will grab attention. Then I would go on to explain that a USPS package was stolen from your doorstep, that you have recovered the empty packaging from the dumpster, and that if the property is not returned that you will file a theft report with the police and give them the package so they can retrieve fingerprints. Maybe this will scare the thief into annonymously returning the property, maybe not. I wouldn't say in the notice that it was a knife that was stolen, your nieghbors don't need to know what the property was, the thief will know.
I don't mean to be a jerk, but I don't see how this was the sellers fault. When a person buys something, it is either up to them to specify the shipping method and shipping details or, by not specifying, they are by default agreeing to leave those details up to the seller. Either way, the buyer is responsible for deciding how the package is shipped. And if the package is not shipped according to the buyers instructions, then the buyer has a justifiable grievance if something goes wrong.
If I recall correctly, shipping companies offer the option of picking up your package from their shipping office (to avoid situations like this). But you have to request this option. Also, you have the option of requiring a signiture for delivery, of course if the package is delivered to the wrong address than this offers little protection against theft. But the signature of the person who signed for the package would be on file.
I'm sorry to hear about your loss. Unfortunately this may prove to be a very expensive and painful learning experience.