Without being too specific (ie 'Oh just go to x in y town')

I was wondering if anyone could help me out with re: to buying spells. I understand that Court Wizards in the towns have spells and I've found all the people at the College who sell spells but I was wondering how it decides which spells are available?

Like one time the conjurer seller had 'summon flame atronach' and then the next time I went he had 'summon frost atronach' - is this just based on my skill in conjuration?

And without being too specific - are there 'other' people selling spells around that I should try and find? Or it everything available at the college/court wizards?

From what I can tell, it's all based on your character's (overall) level. So even if you have 100 Destruction at Level 10, you won't be seeing those Master spells popping up for some time. Court Wizards, from what I can tell, sell all the same spells that are available at the College of Winterhold, too. Don't know about any independent spell vendors.

It's based entirely on your skill and the school the vendor sells. There may be a few things hidden. I think the 100 skill spells might only be obtainable at the college. And a spell or two only available by tome, but I have no confirmation of that, merely heresay.

Can anyone clarify on this?

As to whether or not it goes by your overall level or by your skill in the field?