I'm considering doing some Dark Sun conversions for Myth & Magic. While poking through some of the old 2nd edition Dark Sun stuff, I was struck by the trader class.
Traders are pretty much like Bards, except that they have slightly more practical skills- they learn to buy and sell, appraise and bribe, etc., rather than just learning how to play the lute.
So here's how we make a rough (really rough!) Dark Sun Trader from a Myth & Magic Bard:
Weapons, attributes, armor, and other core features are all fine as they are. When in doubt, assume that the Trader works just like the Bard in terms of progression, HP, etc., until proven otherwise.
Bardic Knowledge: 'Trader Lore'. This ability makes perfect sense for the Trader, since they would be travelling around and hearing rumors, sharing stories with sources, joining caravans, etc.
Bardic Performance: Swap Performance for Diplomacy. Bardic Charm becomes Fast Talk and reflects the Trader talking his way out of sticky situations. You might even allow the Trader to offer bribes to receptive audiences to improve his chances of success. Counter Song becomes Bid Calling, and now relies on the Mercantilism proficiency instead of Performance. Traders, through long years barking orders and working auctions, have learned how to disrupt audible spellcasting through carefully timed and well-projected shouts and hollering (ever heard a live stock auctioneer in the South?). Inspire Allies becomes I Give the Orders- Traders generally are in the habit of coordinating the movements of drovers, porters, and other lackies and are able to effectively coordinate the movements of their allies in combat, or at least irritate the enemy with their constant hollering and complaining.
Rogue Skills: Replace Decipher Script and Perception with Mercantilism and Appraise.
Arcane Spell: Here we get into deeper water, as arcane magic is quite rare in the original Dark Sun setting. For my purposes (since psionics get on my nerves) rather than try and put together a complete psionics system for my campaign, I'm just going to alter the background of the setting to be more open to magic (though it is still quite mysterious), going for more of a Barsoom/psuedo-Vancian feel. In that setting, it seems not unreasonable that Traders would come across the odd tome of arcane lore and try their hand at a few labor saving (or profit improving) spells. Good spell choices here (I fell a random table coming on) would help make the Trader feel a little more distinct from the Bard- probably a more restricted selection of early spells, for instance.
Armored Caster, Activate Arcane Scrolls, & Activate Magic: Since we split the Solomonic baby regarding arcane spells, there's no need to change these abilities around now. Bardic Influence is also fine as-is.
Renown: Seems fine. There are some charts in the Dark Sun books if you want more interesting followers. These followers will be the Trader's agents, acting on his behalf as body guards, buyers, clerks, etc.
Legend: Again, fine as is. When the Trader dies, his heirs become filthy rich members of the permanent oligarchy and he gets his face on a bank note and a university named after him, instead of some crappy song.
At the upper levels, the Mercantile activities rules from Adventurer, Conqueror, King System will be your friend.