There's a lot to process here.
Where I live, UPS will not accept any kind of firearms or ammunition for shipment. They will deliver, if it comes from somewhere else, but they won't ship. I tried discussing this with them one time and ended up speaking with a local manager who said it was a decision made by regional UPS management and that was that. I'm not familiar with USPS rules regarding shipping. That leaves FedEx.
I shipped two long rifles to Track of the Wolf via FedEx for consignment sale 4-5 years ago. One arrived broken. I had taken pictures of the guns to send to Track, so that was fortunate, and the people at Track were great. They did tell me I could have done a better job packing, but said the packaging I used should have been adequate. Also, the local FedEx manager was really good. The upshot was that FedEx awarded me a settlement amounting to about 2/3 the value of the rifle. I took it, and they kept the rifle. Lord only knows what they do with the stuff they keep, but they would not award repair costs and return the rifle to me. It was all or nothing.
This spooked me. I have several guns I would like to sell, and I now know better how to package them, but I've been put off by the costs and hassle. My hat's off to those folks who pack and ship frequently... I guess the more you do it, the easier it gets.
To the OP, thank you for naming that seller. They could and should have absolutely done a better job. If you are a buyer, you are at the seller's mercy. A couple of things I would suggest would be to speak with them before shipping and express your concerns. Get full value insurance. Request photos before shipping, or save photos from the seller's website. Try to avoid the pre-Christmas season.
Specifics about packaging have been covered here many times. Take photos of the gun before shipping, insure for the full value, and be truthful about the package's contents. Try to avoid the pre-Christmas season.
Good luck!
Notchy Bob