Possibility of USPS employee stealing my package?

Well you see the title and now my story which I will ask your humble opinions.
Last Wednesday I ordered an iphone from an online service called swappa.com. I have ordered several phones from this place in the past years without incident.
This time my package shows up on Saturday per my tracking. I sign and my mail man (lady) hands me what feels like a very empty box (which looks stepped on on one side) This was sent in one of those priority flat rate boxes about the size of a VHS tape so there is no weight listed.
I am immediately suspicious but the mailman has driven away immediately. I go in and film myself opening the box and inside is a cushioned pouch which looks sealed (origin flap untouched) but the bottom has been opened and the entire thing has been re taped up. Inside this pouch is no phone but instead a used bakery wrapper from someone’s doughnut.( I am not kidding - this is like a FU move)
I wig out and contact the seller via swappa. He swears that he sent it out directly at the post office and I contact swappa.com “service” and a couple of their service guys ask for more information and pictures. I send everything to the site including the video I made of myself opening the package.
I am still awaiting some resolution from the seller and swappa. (either he didn’t put in in there or it was stolen enroute)
Today I go into my local post office and want to file a complaint for an empty box which should have been an expensive phone which was handed to me when I signed. They immediately start asking if the box was “sealed” I say it appeared sealed but since the top - where it clasps - was partially crushed it is hard to tell. It opened pretty easily to be honest.
I go up the chain and speak to a manager who tells me again that there is nothing they can do since the package was sealed. They are implying that since it was sealed then it was impossible for anyone on their staff to have messed with it during delivery.
I am persistent so she goes back and speaks to the person who gave it to me 4 days ago.
She comes back and says that the mail woman “remembered it having weight and not feeling like it was empty” when she gave it to me after I signed. (really she remembers that one package?)
Now I am not an idiot but they apparently think I am.
I left so my question is this?
What do I do at this point? i am still dealing with swappa and the seller (who has not answered any of my message in two days - suspicious?)
Do I still go online and file a stolen package report thru the post office (this seems like the least I should do but I was told by my USPS that the seller should be the one to file this)
Should I file a police report? How is this done? do I go into the precinct and do this? Again I never received anything physical so maybe that is dumb?
Sorry for the questions but I feel like I am already screwed in this situation.
I paid for this via paypal if that makes any difference.

FYI
I have a friend that used to work for the post office - up to supervisory levels - here in the DC area for over 35 years and retired 3 years ago. She looked at the pictures and heard my story and immediately gave her opinion that the phone was stolen by someone in the post office along the route. She claims that the post office has turned into a crappy job of late and the quality of the pay and workers hired have fallen. This is one of the reasons she retired when she could.

Do you think Swappa.com has a recourse to cover the cost? Insurance?
I agree the USPS was where it most likely went missing. They’ll never agree tho’.
Can PayPal possibly help?

Goodluck.

ETA, the seller didn’t insure the PKG?

As someone who works in the USPS currently here’s my advice.

Contact USPS directly via phone because Priority Mail packages automatically come with something like $125 automatic insurance coverage precisely for mail that was damaged in transit, that’s one of the reasons you ship priority mail in the first place. Also contact seller and have them submit a claim to to USPS since you might need both statements for the claim though you alone being the receiver should be good enough for a start.

As for the description of the package it seems more like the package got either damaged in a machine or damaged in transit, more likely by a much larger package falling on it and causing the box to burst open. I don’t think anyone would steal it because how would they know what’s in the box in the first place? However it would still be on us because mail that’s damaged in transit should have been seen in one of those DAMAGED IN TRANSIT plastic bags we have. I assume the box got damaged, the phone fell out, somebody else saw the damaged box and without looking into the contents resealed it back up and sent it on its way.

Thanks for the replies.
Beckdawrek - the seller did insure the package but I have no idea for how much. He has been strangely silent over the last 2.5 days.

Asuka,
I suppose its possible that someone in the USPS saw the package damaged and open and then completely resealed an empty packing piece. But there was nothing along the lines of “damaged in transit” that would lead me to believe that the person who may have done that didn’t follow the policies. The box was as light as a feather and the inside packet only contained the doughnut bag which was feather light and not noticeable. The weird part about that would be the fact that they taped the entire inner pouch but didn’t bother to fix or try to fix the outside box at all. There was not a hint of tape on it.
will try and call the USPS.
Is there a particular number I should start with besides 1-800-275-8777?

thanks again.