Most of my bayonet collection has come from being bought with the gun that it attaches to. A lot of bidders on auctions tend to ignore the fact that they come with a bayonet so you end up getting the bayonet for free.
Nothing like that, we call those relic condition. Usually available for a good price online. Not a fan of the restoration RU-vidrs who do bayonets, in my opinion they ruin the authenticity of the item. Plus it's amazing how there seems to be rust on wood in their videos, I suspect they coat them in something that looks like rust to make the transformation more dramatic
I've never tried Facebook marketplace (mainly because of all the scammer stories I've heard). Any particular trustworthy groups/sellers you'd recommend?
Two of the groups are pretty well vetted, "THE bayonet collector group" and "INTERNATIONAL BAYONET ASSOCIATION" but they are not dedicated sales groups. Generally I wouldn't consider buying from someone unless I can see a history of sales and good reviews.