This is likely where the issue is. Most of the people “trading” are scammers using illicit gold. They got your real gold, and you got the dirty gold.
Allowed? Technically allowed. However these trades are unsupported for a reason - they can go bad very very quickly and are super risky. Never ever take gold like that from someone you don’t personally know and trust.
You can appeal, but if the gold was in fact illicit gold from a gold trading account, it will likely be upheld.
Keep in mind, the accounts they use are often stolen accounts gained from phishing scams using in-game messages (go to x website to prevent being banned!), or email scams. Even stealing “customer” accounts from their gold buyers. The account holder reports it stolen and as part of clean up everything gets cleaned - including where the stolen gold went.
I can’t say that is what happened to you for sure, but it is likely.
Edit: Found the Support article!
The exchange of gold or items with another player on a different realm or faction is entirely at your own risk and is unsupported. These types of trades are often associated to the trade of real money for in-game items/currency, which is against our policy, and accounts found associated to these services will have gold removed as well as other possible account actions including up to account closure.
While we will review reports of scams between legitimate players and take action against scammers to remove them from our game, losses will not be restored.
We advise to avoid these types of trades unless you know the other player closely and absolutely trust them.