Well, for most reptiles/snakes the general rule that many breeders follow is not to sell until they have had their first shed and 4-6 consecutive feeds, so being sold after only one feed would, in my opinion, be too early.
Not really sure about weight wise - when we got our young female she was around the 80-100g mark, but it should really go more on age and feeding than weight. I would advise against buying an animal that has only fed once - ball pythons in particular are known for being fussy feeders at times, and babies can be difficult to get established on a regular pattern, so let the breeder do the hard work and then you should take home a nice healthy baby.
As for your last question, I'm afraid we're in the UK so can't help you there!
