I think you've heard it all now. The 'M' on the crossguard is an immediate give away that it's not SS, but an SA dagger part. The blade looks completely fake to me, with modern laser etching and strange surface polishing... not a trace of any cross graining.
It would be really nice to see more of the grip, especially the runes button. The eagle looks to me as though it has been crudely pinned or glued into place, and the wood has that slightly porous look to it that should be a red flag. As I said, it would be good to see more. Also of the scabbard.
Thanks for sharing the info on who sold it. How much was the sale price? One of the best ways to spot a fake when you start collecting is when dealers offer a dagger for sale at a massively discounted price.
You said something about him claiming this has original parts... well that means that the dagger isn't original, it might contain one original part and the rest could be a few months old.
This nonsense happens to most of us when we start collecting, and it's a valuable lesson. Please don't give up the hobby...