Ok so I've finally figured out a few things about the npc traders. and because it was so difficult I'm going to make an faq answering a few of the problems that I had while making my first trader. If any of this is wrong feel free to correct me and I'll fix it.
How do I get my own npc trader?
this is a two step process that costs $1000 in slimes.
1. The first step (which costs $750) is to use /npc create [name] (text). the (text) is optional, it just sets the text of what the npc says when you hit it or walk close to it, this can be changed later with /npc set [text] and /npc add [text] (adds new text to the list of things the npc will say). This npc has no function other than to stand around looking like whoever you named it after and saying what you told it to say.
2. Step two costs $250, and is accomplished with /toggle trader. be sure to select your npc by right clicking it before doing any commands about it, but once selected the npc will stay selected for multiple commands. Once your npc is a trader, you can set it to buy and sell things.
Where can I find a list of commands for the npcs?
right here. :
http://citizens.wikia.com/wiki/Commands
Why wont my npc sell anything to anybody?
To be honest I'm not sure why this happens but if you just set your npc to sell something for a certain price (/trader sell 344:16:0 266:1:0) it still wont accept currency from people unless you tell it to buy the currency from the people at the same rate (/trader buy 266:1:0 344:16:0). the examples I provided are the codes I used to make my npc at spawn sell 16 eggs for 1 gold. when I just had it on sell it would tell people that it wasn't buying gold at the moment. so I had to set it to buy gold for 16 eggs, and at the same time sell 16 eggs for 1 gold. now it works like a charm
Correction: I've only had to do this once. it works fine with just the /sell promt now. but if you do have a problem of it not working try the above.
How do I know if my npc is working properly?
Unfortunately as the owner of the npc you can't test it yourself. what I had to do to test my npc was give someone the gold to buy eggs and ask them to attempt to buy eggs. when it didn't work I would ask them what it said the problem was, and attempt to fix it until they were able to buy eggs at the desired rate.
Can I sell an item in stacks, and individually?
As far as I know it's impossible to have one trader selling the same item at different rates. I attempted to have my trader sell individual eggs at 5 slimes per, and stacks at 1 gold per. unfortunately when I tried to add the second price I got an error message saying "this trader is already selling eggs." my best guess for how to do that would be to put up a [trade] sign next to the trader with the second rate on it.
Why isn't my npc talking when people move near it?
first of all you need to use the command /npc talkwhenclose [true/false] to set whether you want your npc to talk to nearby players or not. Also try not to move your npc a lot. I've found that when you move your npc you need to reset it's texts and also tell it to talkwhenclose again. but after moving my own npc around a lot it doesn't seem to be able to talk at all anymore.
What if someone confuses me with an npc I made of myself and punches me?
Punch them back. Let them know you're real, and that it hurts to be punched.
Or just change your npc's name to that of someone you'd like to see get punched.
EDIT: So at the moment I'm having problems with my npc changing the rate at which it sells on its own.
Atm, I'm not sure how to fix it but I'm working on it and I'll let you know when I do.