Not-So-Legal Real Money Trading in MMOs
95+% of all MMOs out there are going to have an official policy against real money trading (RMT). If they officially support it in-game, it adds a layer of complicated and expensive regulation requirements. If they look the other way at grey-market offsite activity, it introduces inevitable complaints from the mainstream player base and potentially wrecks the in-game economy. That's why the overwhelming majority won't ever bother.

If it's not already listed in the Legal MMO section, assume that the publisher does not allow the selling of accounts, gold or items in any given game.
It's important to note that selling your MMO items or account is never illegal under local/state or federal laws here in the United States, because making laws about things like that would be trifling and silly. It's simply against the publisher's terms of service (TOS) that you agree to when you create an account, and the publisher retains the right to strip items or ban your account (usually the latter) if you're doing it. I cannot speak to other countries and their systems of law. 

How to Sell MMO Accounts and Items

Basically, all you need to do is find an interested client, reach an agreed purchase price and complete the deal. Of course, the devil is in the details. Especially considering this is a totally unregulated "grey" market. 

There is a certain inherent risk of ripoffs in an unregulated market such as this that just comes with the territory. You can attempt to reduce the risk somewhat by using a middleman site dedicated to this sort of thing. Several of these exist including:

- G2G 
- PlayerAuctions  
- EpicNPC
- MMO Auctions
- OwnedCore
- PlayerUp
- Sythe
- Marketplace

What these sites will do is provide a layer of security in terms of either the buyer or seller pulling a runner and ripping each other off. This usually comes at the cost of a small percentage of the purchase price paid by the seller. 

This doesn't protect you from account closure by the publisher in any way. You also need to do good research on the middleman site to verify it is reputable, as nothing is protecting you from THEM pulling a runner either. 

Some sites will simply buy your account/gold/items directly from you (to resell on their own). Consider this the "pawn shop" method. It's the quickest and safest, but guaranteed they will lowball you on the price. 

Some of the P2P sites listed above have this function, other sites with a more direct focus on this model include:

- Account Warehouse
- The Game Supply
- USFine

The most profitable way would be to drop the middleman and make a direct arrangement, but this is risky on both sides. The buyer could get ripped off if the seller logs back into the account after the sale and claims it again, the seller could get ripped off if the buyer does something like a chargeback on PayPal. 

If you want to go this route, though, you just need to head to anywhere interested gamers might congregate. Forums, social media, even Craigslist. Some forums have sprung up to take some of the risk out using a "reputation" system of a sort, such as Reddit's GamingMarket

MMOs With an Unofficial RMT Market

Pretty much any MMO with a decent following will probably have interested RMT buyers. Trying to keep a detailed sections on every game out there would be insanity, so we're providing simple links to the most popular games with some helpful resources. 

World of Warcraft (PC, Mac)

EA Sports Games: Madden, FIFA, NBA Live, NHL ( PC, Mac, Consoles, Mobile)

* How to Play: One-time retail price for PC/Mac or consoles, or free-to-play mobile titles like Madden Mobile 
* FIFA coins guide
* Madden coins guide
* NHL coins guide

Guild Wars 2 (PC / Mac)

* How to Play: The core game is free-to-play; the expansion, Heart of Thorns, is a one-time purchase
* New player guide
* Gold farming guide

Star Trek Online ( PC / Mac / PS4 / Xbox One )

* How to Play: Free-to-play with some caps on money and ship elements (optional monthly or one-time "lifetime" subscriptions available). "Cash store" allows you to pay to skip the grind for certain ships and other items. Can play through Steam.
* Guide collection

Champions Online ( PC / Xbox 360 )

* How to Play: Free-to-play with optional paid monthly subscription. F2P has some restrictions like a more limited inventory and inability to use vehicle mounts. Can play on Steam.
* Beginner guide

MapleStory 2 ( PC )

* How to Play: Free-to-play with optional "cash shop" purchases to skip grinding and customize character appearance. Play through Nexon Launcher or Steam.
* Guide collection

EVE Online ( Windows / Mac / Linux )

* How to Play: Free-to-play with limits on skills and ship types available. Optional monthly payments available to unlock full features.
* An example of how in-depth the game can get

Final Fantasy XIV: A Realm Reborn ( Windows / Mac / PS3 / PS4 )

* How to Play: Requires a one-time purchase; available on physical game disc or as a digital download
* Starter Guide

The Elder Scrolls Online ( Windows / Mac / PS4 / Xbox One )

* How to Play: Requires a one-time purchase, and then there is a paid subscription option. The paid subscription gives you a number of game advantages and free access to all of the DLC packs, which otherwise have to be bought separately.
* Game guide collection

Lord of the Rings Online ( Windows / Mac )

* How to Play: Free-to-play with the ability to unlock everything by using earned in-game currency, but a monthly subscription and expansion packs are available for purchase to skip that grind. It costs about $50 USD one-time to unlock everything, and then ongoing monthly subscriptions just give various perks.
* Collection of game guides

Path of Exile ( Windows / PS4 / Xbox One )

* How to Play: Free-to-play for most game content. "Cash shop" stuff is mostly cosmetic but there are some upgrades that hardcore traders may want.
* Beginner's guide

Black Desert Online  ( Windows / PS4 / Xbox One )

* How to Play: One-time purchase price, though there is a one week free trial.
* Guide collection

Runescape ( Browser / Android / iOS  )

* How to Play: Free-to-play through a web browser or app, with optional monthly subscriptions. Free players are confined to a limited map consisting of about 20% of the game.
* Collection of guides 
* Why old-school Runescape gold is still prized   

Granado Espada ( Windows )

* How to Play: Free-to-play with all sorts of optional cash shop purchases
* New player guide  

Pokemon GO ( Android / iOS )

* How to Play: Free-to-play mobile app with a cash shop that mostly sells potent bonus items.
* Guide collection  

Clash of Clans / Clash Royale ( Android / iOS )

* How to Play: Free-to-play mobile app with optional cash shop items to help skip various grinds.
* Guide collection  

Roblox  ( Windows / Mac / Android / iOS / Fire OS / Xbox One )

* How to Play: Free-to-play with optional cash shop cosmetic items.
* Roblox Master Builder Guide 
* Roblox Complete Guide

Fire Emblem Heroes ( Android / iOS )

* How to Play: Free-to-play with cash shop booster items that are mostly needed for high-level competitive play.
* Beginner's guide  

League of Legends ( Windows / Mac )

* How to Play: Free-to-play with a cash shop for optional items.
* Beginner's guide  

Dota 2  ( Windows / Mac / Linux  )

* How to Play: Free-to-play on Steam with all in-game content available without payment; cash store sells cosmetic items a la Team Fortress 2 and CS:GO
* Beginners guide