As far as NCR goes, it's quite simple with traders:
Buster (the midget): normal value for weapons and ammo, cheap buying and expensive selling of other items
Sha Enin: normal value for weapons (not sure about energy weapons), armor and random stuff like fruit, cheap buying and expensive selling of drugs and medicines
Mira and Dusty: normal value for weapons with exception of energy weapons and random stuff, cheap buying and expensive selling of armor
Doctor: normal price for drugs and medicines, cheap buying and expensive selling of everything else
Iguana Bob, the farmer and the FLC guy seem to depend more on your character build than the rest.
Especially charm, but also barter and speech are probably more of an influence than at the other traders.
I'm not sure about this as prices seem to have changed for them with the last update.
That's as far as I know, others might be able to to add more info.
Edit:
Confirmed the fact that Mira and Dusty sell armor expensive.
