I posted this on the bnet forums, silly me, so I'll post here instead. After trading unid items, I experienced my first d3 scammers so here's a guide to prevent some heartache.
Trade scams can net much more gold than bots and blizzard doesn't appear to want to fix the simplistic trade interface. So here's some tips to avoid being scammed.
Let's start with the methods, followed by strategies to counter these methods.
Method 1:
The scammer asks you to move around your items for any reason. You oblige, click on an item, the item is on your cursor, just as you are about the click on another slot, the scammer closes the trade window and you instead click on the ground and drop your item, allowing the scammer to pick it up.
Solution: never agree to move around the item, as there is no reason to, no matter what you are told by the scammer. You will probably be pressured to do this as the scammer will tell you he will leave if you don't. This pressure is significant if the scammer is offering you more than market value for the trade.
A variation to this scam is to close the window just as you are about to put your item into the trade slot right after engaging trade. Use shift-right click to put your item in trade instead to avoid having the item on your mouse cursor at all.
Method 2:
The trade is nearly finalized, you are about to click accept, then the scammer types something, you read it, glance back at the trade window and click accept. While you were looking down at chat, the scammer removes a zero or switches items with a script written to do this very quickly.
Solution: Never rush to complete the trade. If something happens when engaged in trade (other than trading), it was probably meant to distract you. This may be something like a person joining a game, the scammer chatting, giving you more items, resetting trade multiple times to waste time in order to rush you.
Method 3:
The trade is nearly finalized, everything looks good, so you move your cursor to the checkmark, are about to click accept, but the scammer uses a script to remove the item / gold / a zero in milliseconds, faster than your brain can process what's happening, and you both press accept at the same time.
Solution: When the trade looks good, don't immediately accept, but wait a while, look at the loot / gold amount again. This scam works only when you see that the scammer put up his loot, you have no reason to think you are being scammed, so you hit accept. It probably takes you like 2 seconds to come to the decision that the trade is legit and accept, and that's how long the scammer waits until changing his loot.
How to avoid being scammed:
Don't rush the trade.
If something, anything, happens when both parties have the trade finalized, look at the loot / gold again.
Wait a while to accept the trade.
Always shift-right click loot into the trade window. Don't move it around.
If you are getting a great deal, don't disregard the above tips even if it will cost you the deal. That great deal with strings attached is a scam 100%.