I am sorry to say this, but there are quite a few modern reproductions of yataghans made nowadays in Bulgaria, which have similar black horn hilts and brass fittings. Often the bladesmiths who produce them create them according to their own taste without much regard to authentic styles. As the seller is one who sells antiques dug in Bulgaria, there is a possibility that you have an authentic blade that has been given to one of those contemporary bladesmiths, who rehilted it and created a scabbard for it. Here are pictures of a modern yataghan/sheppard's knife I have, in order to show you some similarities:
Of course, I can be wrong, and even if I am right, this does not mean that you need to be unhappy with this yataghan. After all, the blade appears to be authentic and quite nice, and if you do not like the hilt and the scabbard, you can easily get it rehilted in a more traditional manner. Actually, I would be more than happy to have this yataghan for its blade, and if you acquired it because you liked it for one reason or another, then the question about value is irrelevant.