The compelling reason to still use forums for trades is for record keeping. If I want to know how much I sold each of the items to see my net gain in gold, I can just go back to the threads, whether it is archived or not.
However, the auctions system doesn't save anything older than one month. It makes it a pain, since I have to either write it down (how much I obtained, stats) instead of just using search for threads I create in trade forum.
The funny thing is someone that bidded (and won) on an item I put up for auction hasn't picked it up for a month and the record is erased anyway. That's a shortcoming right there. I should've left bad feedback, but too late now, I cannot leave feedback at all.
Also the forums tend to be faster than the auction system and you can trade.
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Just a FYI of some of the auction system's shortcomings.
Auctions = bad for record keeping
LifeInfusion