On Arizona Customs three like that (Large plain Sebenza) have sold since 2016 at $895., $950., and $1000. (in 2020).
I'd say you have something rather valuable there. Between the rarity, the quality of Butch Vallotton's work, and the fact that he is deceased and won't be making any more, I would expect the price to be higher than what I mentioned. If I were you, I might wait awhile and see if others are sold and see what they sell for. Wouldn't you hate to sell it for $1000 only to see shortly after that they are selling for significantly more.
On a related note, I'm just now made aware of Mr. Vallotton's passing. This is a real loss to the knife world, and in particular the switchblade world. He is one of the all-time greats. I've admired, and marveled at his work for several years now.
Now that I think about it, if the knife were mine I'd likely keep it and treasure it.