is every temple a different temple guild?
Yep. You can only join one, because in the bay area, there is no 'nine' just my aedra is best and everyone elses svcks. Quest structure and skills though are the same, it's just a way to RP a follower of specifically Akatosh, Julianos, Arkay, etc. Also, this aspect of the temples are made fun of in the play "Fool's Ebony."
how do the DF guilds work.. do you ascend up the ranks ala oblivion to become master or what?
You do lots of random quests, and depending on your skills and guild reputation, you climb the latter when you ask for it. Some guilds will ask you do to specific tasks before they'll promote you though. Also, if you neglect a guild you joined, they'll begin to hate you.
If the quests are randomized.. does that mean the ones I get no-one else will have ever got?
No, people will have done them too, it's just that you task is randomized. One day you are sent to kill giants, the next wereboars, then back to giants, etc, etc. But, you can decline tasks till you find something that fits your fancy.
or does it mean I get the same text quest, but the dungeon will be randomised?
Can be this.
Why cant i sell all sets of armor to general stores? they bought my dagger but not my surplus swords and spare helmets?
Go to a smith, they'll buy weapons and armor. These merchants are much more picky about what they buy, more so than MW and OB.
to whom do I sell my helmets to? is it a case of mercantile skill?
Sell them to armor smiths, mercantile is for how much money you shell out/make from items you buy/sell.