Well here goes yet another uppity answer

I would pass on the does. It's one thing for a planned mating to be doeling back to her sire, it's well another thing to breed them "because there was no other buck", and then sell them as breeding stock. Now if you want them because the does themselves are nice, and you are far enough along in your goat breeding to be ABLE to kill at birth a kid who has a birth defect, mouth deformity, extra teats etc. than no you shouldn't buy animals this closely related, nor should you breed close until you can. Culling is spelled killing (butchering if you have something you can keep until butchering size), not selling to unsuspecting others. Are you also well enough versed in bloodlines to know if this buck is actually unrelated to the does themselves? I would want to see the extended pedigrees of these animals. It's simply no way to start a herd.
Yes I do breed son to dam, dauther back to sire or grandsire, and even a full sister/full brother mating this year, the difference it's planned, I know the bloodlines very well (some I have been dealing with for 15 years, and wanted to condense down that genepool of my two best udders this year. I also will not be selling them to some unsuspecting person, they will stay here, and hopefully become showstring hopefulls.
If you do purchase the does, have another ND breeder help you find a good buck for next year that will improve these does, not just get them bred. Vicki