Well, definitely a red flag that he knew legs were a problem. I'm not real familiar with the NPIP program, but that doesn't seem like something that should be acceptable. I just wouldn't buy from him again.
Bringing in 1-3 day old chicks is usually ok, that is usually what you get from feed stores etc. It's the older birds, that were somewhere else long enough to be exposed to stuff that is usually a problem. Local people are hit or miss. I generally avoid it unless I know them well and know how the birds are/were kept. Some people are very honorable and good, many are not. Hatched chicks are usually shipped at a day old, so they aren't really likely to be exposed to most stuff. That doesn't rule out genetic issues, since you don't know the parent stock, but major genetic stuff the hatcheries generally work to avoid. If they sell bad birds they are not going to stay in business. And many of the larger ones stand behind their birds and will refund or replace when there are issues that were not caused by the recipient. They will also often guarantee sex and refund or replace. I don't like shipping chicks, rarely do it. But sometimes it's the only way to get what you want or need. I did it this year, lost two in shipping and one failed to thrive, so 3 out of 16. I was refunded the shipping losses, didn't claim the failure to thrive. But it was a breed I haven't been able to find anywhere in decent driving distance. I intend to breed them here now, and won't ship again unless/until I need to diversify genetics, that will be several years at least. There are a lot of threads on here where people got birds from someone local and ended up bringing in disease, sometimes it's heartbreaking. Sometimes it works out ok. Just part of all of the many things to consider when bringing in birds.
I hope it's not MG/MS and it's only genetics. As bad as that is for them, at least you will know it won't affect your other birds.
he was culled. 
