Fallout's layout, but I liked the Mercantile skill in Oblivion 'In theory' FAR better over Barter in Fo3.
Also if there is a mercentile skill it should also count the size of a transaction not just the number of transactions, if i sell 50 steaks in one go, it should give me similar experience to selling 50 steaks one at a time. Not one trade vs. 50 trades, that was terrible.
Yes! This always pissed me off. You had to up your mercantile by selling a stack of 500 arrows one at a time, instead of selling all 500 and getting the exp for it all at once. If they did the Mercantile system in Oblivion, with the Fallout 3 layout and set-up, but it counted size not number, that would be perfect!