A Denizen of Leng, a Mercane and a Witchwyrd walk into a tavern.

[the David]

My attention was recently drawn to these wonderful creatures. I can see the Mercane and the Witchwyrd working together as associates, with the DoL as an unlikely business partner. I do see a problem with these creatures. They lack certain abilities that I would think are crucial for merchants in a Pathfinder campaign.

The DoL has Spellcraft, Knowledge (any one), but no Appraise or Detect Magic.
The Mercane has Appraise, Knowledge (arcana) and Spellcraft, but no Detect Magic
The Witchwyrd has Appraise, Detect Magic and Knowledge (arcana), but no Spellcraft.
None of them can identify magic items even though they are all supposed to trade in magic items. Now this is all easily solved, but it's still somewhat annoying.

Although they are all able to communicate with any creature that has a language trough Tongues or Telepathy, they don't have a reliable method to contact their customers. Giving them Sending as a spell-like ability would go a long way. This still doesn't give their customers any way of getting in touch with them, but it's a start. The alternative is handing out Scrolls of Sending with every transaction.

Planeshift is rather unreliable as a source of transportation. You arrive 5-500 miles away from your intended destination. This does make an interesting plot hook as the group of merchants suddenly needs a local guide/bodyguard to lead them safely to the location of that dragon they want to trade with. Not having a way to inform the customer that you're late makes this annoying.
Ofcourse both the DoL and the Witchwyrd have access to other methods of transportation that aren't clearly defined. Planeshift is still the cheaper option.

This is my view on these creatures. I'd like to know what other people think of them, and how they fit into your campaign. Are there any other abilities that planar merchants should have that I missed? Are there any other planar merchants that I haven't heard of yet?

[Rajnish Umbra, Shadow Caller]

I don't see any of them as the kind of "Door to door" salesmen that would go around visiting dragons, and more as travelling from commercial center to commercial center, from port to port.

If someone requires their services - Mercane are outsiders, they can be called. Denizens of Leng, too. Witchwyrds can still be contacted via sending, but have to be met the mundane way.

Still, I assume most of the "how do they..." questions would be answered by "they don't", because, as mentioned, I see them more as travelling from market to market - if you are important enough to warrant their personal attention, then you should be able to contact them on your own.

[the David]

I'm not sure how you got door to door salesmen out of this. (It's a funny idea though.)
I imagine that they only appear when someone needs their services. Traveling from one market to the next wouldn't seem very profitable to me. I guess it depends on the availability of magic items in your setting. Considering that the average person earns one gp per day, I'd say it's pretty hard to find a magic item on the market, which is why I prefer the on demand approach.
Then again, Pathfinder isn't exactly known for its realistic portrayal of the economy. What do I know?

Edit: And just for a quick frame of reference on the on demand approach, if a player wants a cloak of resistance +1, they will have to go to Gnando the gnome tailor. If they don't specify any details beyond "cloak of resistance" they will get a perriwinkle cloak with a duck print on it for not roleplaying.
The dwarven brothers Eitri and Brokkr make custom weapons and armor in a similar manner, except that they use the dynamic item creation rules.
Mercanes and the like fit in by supplying any items for those who can contact them, which is a very limiting approach to doing business.

[blahpers]

Mercane don't tend to frequent village markets. Think bigger. Like "Sigil" bigger.

[Meirril]

Honestly most big scale and moves a lot of magic items kind of merchants would develop an organization. Depending on who and what they have available to them would change the way everything works, but there would be a lot of middlemen involved that take orders and pass them on to beings that hold the items themselves.

Let me give an example. Say a group of enterprising Outsiders wish to set up a magic item exchange. They contact various beings they know that could create magic items for them. They make deals with various beings that have large amounts of treasure around and aren't too particular about exchanging bits of it so long as they get a profit from each trade. They create vaults shielded from scrying and various thieves and extra dimensional travelers. They set up a network of agents that can all make familiar contracts or bind a minor outsider that can teleport themselves + 50 lbs.

Now a customer visits a 'magic shop' in a large city. The owner is an 8th level bard that specializes in negotiating deals. He has an imp familiar. After the deal is made he gives the imp a copy of the contract and half of the payment. The imp teleports to the central office of the consortium. There the contract is double checked and fulfilled by an account manager who gives the requested item to the imp who transports it back to its master. The master contacts the customer to arrange for delivery plus final payment.

This could be done by anyone with sufficient knowledge and ability. Outsiders are just closer to being able to setting the business up than humanoids. The outsiders could be the agents, or they could find locals that can work with them. The main key is an ability to move information and merchandise safely and in a timely manner.