1. Give gold weight.
2. Generally enable players to hold less (less loot from dungeons to sell - players must make a decision about what to take and what to leave).
3. Give merchants less gold to spend on random goods peddled by the PC. The vast majority of a businessman's money which is not for consumption would already be in use for a longer term investment - not just laying around waiting for someone to peddle random items.
4. Make it so that merchants pay very, very little on items that they have little knowledge of, e.g. a potion seller does not know if it is worth buying armour from the PC for resale. Thus the general traders would usually be the best to sell poor and average items to (since specialised traders are better able to detect quality), and specialised traders would be the best to sell high quality items to. This adds an extra level of transaction costs to the game.
5. Have general traders buying lower than other traders due to their less specialised knowledge. Therefore, in conjunction with 4, they would be the most convenient to sell to, but the least profitable in monetary terms. This would of course be influenced by Speechcraft (which I believe now includes the Mercantile skill), giving it a more important use.