Good luck man. Most auctions companies will not disclose their buyers or info. It's to protect their own company and the buyers / sellers id. I have asked a few times if they could contact or forward a message for me. Surprising I once got a email back from a buyer. They didn't want to sell the item, but I get 1st shot at it if they do down the line. Premier props is a nasty company in general from payments, shipping, mistakes and office help.
Hope you get a lead somewhere and get your boots back. Maybe have a new set made by the company that did them.
At best, Premiere may be willing to contact the winner and give them your contact information. They've done that before for me in the past, and it's resulted in me actually acquiring an item or two... That was years ago, before they got all shady and became disrespected in the community.
Tell them you are looking for Outlander item # OTLD2359, that's their inventory code for that lot...
It sold for $15, so it's possible that it didn't even go to a prop collector. A lot of times period outfits like this will sell to people that actually want a usable costume, and it's easy (and frequently cheap) to pick up extra/background costumes like this from wardrobe sales. If that's the case, PP getting you in touch with the winner might be your only hope (but he may be more willing to part with them though, considering your history with them).
You might actually have better luck with the sword, as that would fall more under 'prop' for most people... I have a few of them myself actually. Never saw a hero version of Kainan's wicked looking sword (I have a half-sized one though, apparently CNC'd at the wrong scale, and there was a stunt one of some kind sold, but that was all I ever saw). Lot's of other random swords (and just blades) were sold off though... What one are you looking for?
Then there was the type of sword I had, which was well-made and nice looking but quite common, and also used by Kainan (Jim Caviezel) in several scenes. I think the reproduction sword was based on his one, but they all had the same wrapped handles, pommels, and brass beads inset into the hilt. The one you see here in this linked image is pattern welded/damascus, but none of the actual prop swords had this type of blade, except perhaps for Caviezel's hero sword.
Yeah, there were at least two of these that were made of the damascus steel, I have one that the pommel has broken off of, but you can still see the patterns in the steel, and it's sharp as hell... No way this was actually used to swing in the presence of another actor I'd assume, it can hurt people...
The other one was still completely intact with the scabbard and such, so I assume there were two hero swords and they broke one. (Premiere sold both of these with the damascus steel blades)
I have another one that looks just like it, but is made out of cheap stamped aluminum and painted with black paint to give it some depth... Sounds like one of the Valentine ones (although it has a really nice handle/pommel/crossguard with steel patterns like you see on the hero sword, though slightly bent like it was used roughly...)