Trying to differentiate a small to midsize Nanotyrannus and Dakotaraptor is pretty difficult.  It appears that Dakotaraptor teeth are not common and sellers are quick to assign Nanotyrannus teeth to them.   You need to look at all the characteristics before making a determination on a tooth and it may turn out to be indeterminate.    REMINDER we have a limited view of these teeth.   We have not see a complete dentition and most likely there some variations in dental positions.   W