Let me preface my post by saying I don't play a trader and don't know how to play a trader, but I have a pretty good understanding of magic in the DR-universe.

Magic is sort of like the vapor trail left by the Immortals. Holy magic being the "fastest vibrating" of sorts, the most recently-left energy, and progressing downwards from it is life, elemental, and then lunar which is above gravity. So lunar magic essentially is magic powered by large bodies of mass and is related to gravity, highlighting both moving objects around and bending light (such as clairvoyance, shadows, and moongate).

It was said in the initial post about 3.1 that the two sort of domains of moon mages, ability to travel big distances and power of prediction, were to remain in the hands of moon mages. That -is- unfortunate since those are two things a trader would highly value. But perhaps trader magic can be other facets of gravity and manipulation of light, with some limited abilities that blend into the domain of moon mages. The way I see it is 1 - light manipulation, 2 - gravity manipulation, 3 - clairvoyance/mind control, 4 - travel assistance. 3/4 obviously move into the moon mage territory, but in ways I think that could work from a lore standpoint.

1) Simple spells for traders could be to move light to aid them and chance speculate paranoia into a spell that creates light around the trader for a perception buff (Augmentation). Similarly, the ability to redirect light around the trader could make them perhaps appear blurred and harder to hit with ranged and targeted magic (Warding). Clarify gem (utility) would obviously be a useful spell for a trader and fit within this framework for trader magic. Delving into things more esoteric, perhaps the ability to see imperfections in other materials (and have a magical ability to purify them) could be in order. So a trader could increase the quality of various materials (metals, fabrics, bones, whatever).

2) On the subject of harnessing the power of large bodies, a trader perhaps COULD affect gravity and have a significantly more powerful version of ease burden (utility), burden could be made more powerful and moved to a trader spell (debilitation), or the ability to affect a small area of gravity increase the weight capacity of a container (this could be cyclic utility spell; on releasing it, it wouldn't spew things out, but maybe a roundtime on moving until you cast it again or take things out to make it right). Traders could have a spell that increases the gravity of an opponent that would make physical ranged attacks more likely to hit (debil) or have it work on just a specific item and replace speculate coin (targeted magic)with this spell.

3) Moving closer to the border with moon mages, speculate finesse would be venturing into the realm of prediction and quasi-mind control. On a related note, the ability to influence enemies to move and react slower (debilitation) would be a useful spell (of course limited to critters with a brain). Traders would have a big interest in the ability to influence people, too (think, how to win friends and influence people), and a charisma and/or scholarship buff (aug) would be in order. A trader would also likely want the ability to communicate wider, so an ability to boost the range of gweths/albredine would be quite useful.

4) This is likely the most controversial of my ideas, but what about some kind of a limited moongate-type ability. It could be a ritual spell with a significant cooldown. The power of it is affected by your magic ability, and the cooldown is affected by say.. charisma (a way of making it like circle based but not exactly, RP-wise think of it like immense willpower to muster up the mental stamina to do it again). Early on, the trader could move him/herself along with a caravan across short distances like rivers. Later on, across big distances like the gondola or the from Haven to Lang. At immense power, like maybe 500 ranks (magic is tert, remember, so this would be a big deal), a trader could get across to the islands. The cool down for something like this would be significant, so it's likely just good for a one-way quick trip and you might want to find another way back.

I know I made this an immensely, stupidly long post, and as I said, I'm not a trader, but I'm just throwing these ideas out so that maybe someone who is more versed on the subject could tweak these into more useable ideas with what I've said as a RP-backing.