One way to fix the ghost trader problem is a unified auction house.
I know, players in ESO are against the idea, and *** all over it, but I believe that's only because they have a monopoly on it.
Its actually a benifit to all players. All players would have an opportunity to sell thier good and make gold where they wouldn't normally have the chance to do so, and without joining a trade guild that asks for gold in return. The entire "guild needs millions of gold for prime trader" thing goes away in its entirety, and removes the problem of ghost traders as well as guilds that scam you out of your gold then kick you out.
It can also have the benifit of lowering the price we pay for items. With more competition for items, the more the price can drop.
And while the GM's of guilds that have the market cornered on guild traders will hate this idea all away around and upside down (because they have the system rigged and will lose tons of gold.), this is a benifit to every other player.

Will it happen, though? All signs point to no...
In the name of being unique, problems with this system will persist. Greed and higher prices will rule the day, and a ghost trader in every other booth.

No. Because they can be no players, that may not know the value of what they have. It’s incredibly disheartening to come across a new player in zone chat, who is disgruntled and depressed at the fact that they sold away tons of tempering alloys for as low as 5k each to someone who made them believe that they were buying them at their market price. You really can sit back, and say, “Well, that’s their problem for not checking.” Really? Especially when their new to the game, and unfamiliar with things like price checks and all that?

If so, then I have to ask you this. Since when did we as a community find predatory practices like that okay? It was only within the past year or so that people started popping up, and making those posts in zone chat like what you listed. Some of those people who post those types of things even go on to later brag in guilds that are merchant/trader oriented of how they scammed and or tricked new players out of millions of gold worth of items. That’s sick, dude. Seriously. That’s seriously F’d up.

First, you literally can't even get gold upgrade materials until you get a very high crafting level, at which point you will have a very good idea of their value. New players can't even get them, so your point there is moot.

Second, there are guild traders spread across literally the whole world that you can use as a reference point for prices, and only a complete moron won't give them at least a cursory looking-over before they commit to selling something valuable.

Third, the very first thing you learn as you begin to interact with people is to NEVER take anything someone says at face value, PARTICULARLY when it comes to business.

If anyone in this game gets scammed out of a fair trade, it is entirely their fault and actually serves as a healthy life lesson.

On the topic of selling skins: When you are selling a legitimate run where the dungeon/trial is played normally, you are effectively making a business out of being a "Sherpa" for the content, which is perfectly acceptable and I actually wish more "Sherpas" were available so I could find them, team up, and get my skins done instead of having to slog through PuGs looking for competent players to gather. I would also love being paid to be a "Sherpa", since 99% of my PuGs and even most of my premade groups are AWFUL at DLC dungeons and I honestly SHOULD be getting paid for having to babysit them through and explain every single mechanic over and over again.

If, on the other hand, you're GLITCHING the content, that is obviously not acceptable under any circumstances. It feels to me like people are not understanding this crucial difference. There are 3 types of "skin services":

1. Client pays gold to be carried through a legitimate run, all non-clients get a cut. Acceptable.
2. Client pays cash instead of gold. Unacceptable.
3. Client pays in any form for a glitch carry. Unacceptable.

Start understanding the difference, people!