So after answering this question in another thread, it occurred to me that this may be a useful guide for new players (especially those who are totally new to MMOs). Feel free to add anything you think I've missed.
What is gold-seller spam?
If you see any in-game chat where it's someone repeating the same "Go to [x] website to get [x] gold for $[x]," or powerleveling services, or (outside of the forthcoming cash shop) any services at all that involve paying real-world money for in-game currency/items, that's gold-seller spam. It is a scam. In the event that they actually do provide the services advertised, your account will be compromised later so they can use it to drum up more business from other people who don't know any better.
How does this hurt the game?
Gold-selling/buying is not a victimless crime. If the gold-seller is using a bot to farm, they're breaking the game for any players who aren't playing with a bot. If they're just stealing the gold from compromised accounts, they're obviously hurting those players. More importantly, gold sellers rarely, if ever, follow through on their promises. Their sites are designed to take your information so that your account can also be compromised and used to take more people's money. And of course, in any MMO, gold-selling hurts the player economy.
Can't Zenimax Online just stop this from happening?
Short of hiring someone to stand behind every player and slap their wrist when they do something stupid online, no. They can only take action against accounts after they've already been compromised, which is too late for the affected player. 100% of account compromises happen clientside, so it's up to you to protect your information. And just like Smokey the Bear says, "Only you can stop Chinese gold farmers from ruining our game."
So how do I stop this happening to me?
1) Never, ever, ever, ever in a million years should you go to the linked site. Even if you're curious. Don't go. You'll get malware, even if you don't give them all your information for them to later steal.
2) Don't share your account information with anyone you don't know.
3) Have good anti-virus and anti-malware programs.
4) Always, always, always report gold-seller spam and report anyone who you believe is using exploits or buying gold. Players are the first, last, and best line of defense against scams.