get a grindstone and sharpen it yourself
Dunno about ThatRandomGuy's dagger, but often you'll get (cheap) replica blades that are blunt, but the metal they're made out of is fairly brittle and designed to shatter should you try to sharpen them.
I've got a replica Roman Pugio that is supposedly designed as such. (I bought it from a shop in Rome, and found reviews for it later on (as they're mass-produced by a Spanish company for tourist shops) saying it couldn't be sharpened, but I haven't tested that out because it's not worth the risk of it actually breaking)