The first thing that came to mind was either 1) PBFD or 2) another birdie companion had been severely stressed and was plucking the bloody heck out of your baby.

PBFD, or Psittacine Beak and Feather Disease, is a virus which kills feather and beak cells and also attacks the immune system, which causes the bird to be vulnerable to bacteria and can be fatal. I only know about this disease because a Too/Macaw breeder of mine tests for PBFD in all her breeding stock. Therefore, I don't know much about this illness, but chances are your bird does NOT have it. If he did have PBFD, it is most likely that he would have lost feathers on all parts of his body, not just his neck.

Here's a link fore info: http://numbat.murdoch.edu.au/caf/pbfd.htm

A more likely conclusion would be that your baby had been severely plucked by another birdie companion due to stress and/or malnutrition. Many auctions are incredibly stressful, neglectful, and quite unsanitary for animals of all kinds. Chances are, your baby was kept in a bin with a whole myriad of other tiel (and possibly non-tiel) companions. It'd be absolutely normal (well, not normal, but expected) for a stressed bird to pluck its playmates, to the point where the hair follicles are permanently damaged and cannot grow back. I hope this is the cause of your bird's featherless-ness because it will not affect her longetivity or quality of life

Also, I just remembered that there is a 'disease' in Budgerigars that affects the growth of feathers in budgie fledglings. It's called French Moult, but I don't know if it affects other species, like Cockatiels.

Here's another link: http://www.birdhealth.com.au/about/r-polyomavirus.html

Good Luck!