Lyman did not buy out Investarms. Investarms made the Great Plains rifle for Lyman. Investarms is still making muzzleloaders while Lyman is no longer involved in black powder rifles.
As far as quality, there is absolutely nothing wrong with Investarms. I have several but I don't have a flintlock so can't speak to how good of a lock they have. Unless the gun is rough, consider yourself lucky if you get a flintlock version under $450.00.
Read your post again and I see you say in like new condition with a tang sight. With the sight, I would think it would fetch $600.00 if the gun is nice.