The thickness of the blade and the 'waves' are not ideal for a paperknife
There is evidence of 'oldish' damage to the tip of the blade...from legitimate use or abuse ...who knows.
The dagger, IMHO strongly suggests SEA origins, (or European (Spanish ?)made with SEA influences )
It is difficult to access patina when you 'lighten' (brightness, contrast etc) the quite dark 'listing' pictures. I was looking more at the wear and tear of the hilt which suggested to me 'some age'.
If this dagger is/was of some significance, it is likely the blade would have been kept relatively 'clean', ruining the chance of a reasonable patina to establish itself.
Although unconvinced that this dagger was worth the money, I would not be surprised if it does, indeed have ' monatary value'.... higher than the general concensus expressed.
Regards David