This is a problem I've experienced in both WOTLK and Cata and I've recently seen people posting saying they're waiting on their money arriving so thought I'd explain this problem.
When you sell something on the AH it takes 1 hour to arrive but you immediately get a pending mail in your mailbox telling you the item sold, the person who bought it, how much you'll receive, and an estimate of when the payment will arrive.
So around 1 hour later you receive the payment mail into your mailbox and you collect the money. The problem is that the pending mail remains (until you delete it), so if you don't understand what is happening and/or pay attention to what you've collected you think you still have payments waiting to arrive.
I've taken some screenies to show how this issue manifests.
This first image shows that the first mail is pending and is due to arrive at approx 6:41 (it's 6:13 server time).
http://www.rave.pwp.blueyonder.co.uk...AHMailbox1.jpg
The next 2 images show a payment which has arrived, and the matching pending email which is paired with it:
http://www.rave.pwp.blueyonder.co.uk...AHMailbox2.jpg
http://www.rave.pwp.blueyonder.co.uk...AHMailbox3.jpg
As you can see the item sold, the person who bought it, and the amount I sold it for all match. The time it was due is about 10.5 hours before the current server time, so you can see that this arrived a long time ago. If you leave it long enough before logging on the pending mail does appear to get deleted, but I'm not sure how long. I'm also not sure if this happens for everyone or just some; maybe it's addon related (I used postal addon for mail).
In the next image is a summary of the first pending mail on it's own, followed by 2 pairs (well 3 actually, I've only highlighted 2) of pending + attachment mails.
http://www.rave.pwp.blueyonder.co.uk...AHMailbox4.jpg
So if, like me, you sell a lot of things on the AH and use an addon to auto open your mails, you'll end up with a whole load of pending mails, some which have arrived and some which haven't. Before I select "Open All" on Postal I tend to manually go through and delete all the ones which have an attachment above them. This way all the ones which remain pending should be ones which haven't arrived, which can be confirmed by looking at the time on the mail (it should be less than the current server time, as displayed below the mini map).
Tbh this is far more work than it needs to be, as you can actually just delete all pending mails. The ones which haven't arrived yet will still appear as attachments. Here's some more screenies to prove this:
I listed a scroll on the AH and got someone to buy it. Here we can see it's due to arrive at approx 2:17am.
http://www.rave.pwp.blueyonder.co.uk...AHMailbox5.jpg
I immediately delete the pending mail.
http://www.rave.pwp.blueyonder.co.uk...AHMailbox6.jpg
And then we wait until 2:17am ish and it appears.
http://www.rave.pwp.blueyonder.co.uk...AHMailbox7.jpg
Proof that it is safe to delete any pending mails you have and you will not miss any payments.
Also if you have a lot of pending mails to delete and you accidently delete one which does have an attachment, you will be prompted to confirm the delete (at which point you'd obviously select no).
TLDR
If you are having problems with sales from the AH taking ages to arrive this is a glitch and you've probably already received the payment, unless it's within an hour of the sale. You can safely delete all pending mails and know that you have not missed any payments.
