Getting People to Like You
When one's been selling for a long time you tend to gain a reputation. This can be infamy, as with the case of Phatlewts transmog selling character, who is apparently quite renowned on his server as "the guy with all the overpriced greens". However there's also the low end of the scale where you gain a bad reputation, which can come from a few sources:
Consistently massively overpricing items:
"They always charge too much, I know not to buy from them or else I'll be paying too much."
Being surly towards customers:
"A 5% discount? Not on your life."
Over time your handling of sales and conversations can garner you either a positive rep, bad rep, or keep you quietly anonymous, here's a few positive examples.
I had a guy whisper me asking if I could sell him a Dragonling at a reduced price (costs maybe 75g to make on my server, prices were around 600g each), and so I agreed to toss it to him for 400g, and asked what cogwheels he wanted. When we traded I'd pre-gemmed the Dragonling for him and told him not to worry about the cost. Usually this is the point where I'd feel that I completed my good deed for the day (minus the whole 250% of material cost thing) and never hear from the guy again, however about 10 minutes later one of my friends wsp'd me about the guy. Apparently I hadn't noticed the person I was selling to was in a levelling guild that one of my friends had an alt in, apparently shortly after the sale he had told the entire guild to buy from my bank toon if they ever noticed the selling name because "I was a really nice guy.".
Now this isn't something that's always going to happen, but according to my friend there was around 25 people on at the time, that's 25 people that now might be willing to pay another gold or two to specifically buy my auctions if I was undercut. The lesson to be learned: Never underestimate word of mouth.
I can't exactly cover all the aspects of trade, the horrible amorphous monstrosity it is, but I can assume most of us in the gold making business only tend to take to trade chat on two occasions: To advertise a particularly unsought item, or to make a (hopefully) witty remark. When advertising an item it's good to have catchy sales pitch to get people interested besides the usual "Blah-Blah-Blah in AH" that oozes across trade in a constant flow, however too much elaboration can turn people off as well.
Buy the new [Stick]! With this wooden harbinger of doom you'll be scoring critical homeruns all across Azeroth, your foes will run from your glory, all will look up to you as a paragon of all that is righteous in Warcraft. Legions of women and grovelling fans, and - etc,etc,etc.
If your sales pitch goes like this you better be selling the Ashbringer.
[Stick] now on the AH, unique transmog skin that overwrites the current enchant graphic. Perfect for those of you tired of watching the same pulsing weapon glow for hours on end.
To the point and it gets across why they should want it without saying "BUY IT".
The only advice I have for non selling comments in trade is to keep it high class. Witty remarks that aren't particularly biting will maintain at least a positive reputation, and helpful comments will help accrue a "Nice Guy" image.
Say: "I'm sorry sir, I'd love to have a battle of wits with you, but you appear unarmed."
Rather than: "And what do you know about it, idiot."
This is the most intangible of the topics, on the one hand we want to demand a good price and get our monies worth out of the time and effort we put into acquiring an item for sale; even if it was 10 mouse clicks to buy / repost an item, or (the horror) 20 clicks to buy mats, craft and resell. Market prices on high demand items can be hard to raise, you can of course reset market prices a little higher every time, but people are used to seeing a certain price range. If a gem usually sells for 80-200g and suddenly you post them for 750g, you may see a desperation sale or two, but generally you'll immediately get undercut by a large margin from people who think "No one will buy at those prices, I'll sell quick if I post a lot lower!". In this way posting at slightly higher prices (think 5-10%) can gradually get people used to paying / selling for a higher price, however this only applies to high demand markets.
For markets with intangible prices, it's both easier and A LOT harder to change prices. Items that are hard to obtain and have no competition allow you to charge whatever price you want, within reason;
However with items like transmog, if people see "Pimpin Purple Armor" up for 7500g they might be interested, until they shop around a bit and see "Shiny Shoes" and "Kitten Mace of the Fluff" up for 500g. They'll opt to go with the less appealing, but significantly cheaper option in a case like that, but if they look at the other items and see a price of 6000g then the armor doesn't seem like such a big leap.
In this way I can use the transmog market on my own server as an example, with four major sellers present when I first entered the market. Three people with a 50/25/25% share of the market selling almost every piece for 300-750g and one guy posting everything for 2000-4000 (we're talking hideous gear up for an arm and a leg). I started posting items at slightly higher prices and slowly the rest of the sellers started to raise some of their prices, now the 300g or less price range is reserved only for the moth eaten transmog and the general price range for medium to high tier transmog is 500-1500g, with upper tier items priced for 2000+. The prices aren't exactly ideal but this shows how over time pricing can slowly change the norm.
Keep in mind in your everyday activities that although each individual encounter may not make or break you, over time your actions can have a large impact on your business. Be the person that makes people go "Oh yaaa, that guy!" not "Ugh...that guy."