Okay I wasn't going to jump in but I guess I will.
5 months 1 lb, huge warning bells.
I would never consider selling a puppy that size at that age and this one if I were the breeder I would not be selling it at all. It is far too likely that there is something very wrong and that is why this puppy is this tiny. BTW, a 5 month old especially that small, needs to eat oftener than twice a day. Anorexia a common in health problem puppies and documented as a common symptom in LS puppies.
I don't think the weight has anything to do with how much it is eating, something is wrong. This puppy in my opinion, is severely undersized.
There are issues that can be wrong that CANNOT BE TESTED FOR. What are they?
Embryonic or immature kidneys. Neurological problems that may not show up even with a specialist. Other liver issues that a test may not show up, may need an exploratory perhaps liver biopsy which likely cannot be done given the size of this puppy.
Growth hormone issues and those are just off the top of my head.
My bet is also hydrocephalic and I would definitely look for an open fontanel. If it is hydrocephalic, an open fontanel is better because it will help pressure from building up.
This puppy should not be sold. It should be kept by the breeder, wait and see, run tests, do the spaying with your Vet when it would be safe if ever.
If this puppy is sold, my bet would be the Vet bills could very well be astronomical as time goes by.
I think there is something everyone should realize especially in any puppy that isn't quite the norm.
From conception to birth, the dams system is what keeps it going and growing. ONce it is born, its own system now has to support it, it does not have the help of the moms physiology anymore to do that. Over time, which can be days weeks months often the puppies system, if there is a problem, will keep compensating to sustain life. Over time, eventually the probelm will manifest as the body can no long keep compensating.
JMHO and yes I am a show exhibitor/breeder with absolutely nothing for sale until I have no idea when. My website totally revamped should you choose to visit and read lots of info on yorkies is
www.loribenyorkys.com