Is there any differences in price for selling your items to merchants that specialise in specific items? EG selling ingots to Blacksmiths would result in getting a better deal then selling it to a merchant who specialises in alchemy or spells. Or is it just an overall shop that that gives you better prices all round?

For the most part, it makes no difference. I have noticed at least one merchant, Enthir in the Mage's College, who has inflated prices.

There is a speech perk that gives you better prices when bartering with the opposite gender. You'd notice better prices when bartering with male/female merchants then.

I'd just sell items to whoever has the money to buy them. There is no "disposition" like in previous ES games.