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I literally just filmed a video for the forum on my favorite armor items and how I avoid getting the dreaded selling error in the process of listing them. Today I'd like to talk about the second part, namely avoiding the "Error Attempting to Sell."
Let's say you put orders in for armor in quantities of 50-250, as I do, and after a long night of drinking most of your buy orders have completed. You need to stick to very specific types of armor in order to complete your orders so quickly. Anyways, regardless of how much armor you bought, you will often fill your bags quickly and have many more items waiting on the trading post. If you sift through all your currently sell-able items and start posting them all at once, then you will get that wonderful error message.
Instead, you should pick out the highest stacks in your bags and sell those. Maybe the top 4 or 5. Once done, go back to the pickup window and grab some more items. Now the smaller stacks will have grown and you'll have 4 or 5 new stacks to choose from and sell. Post the new highest stacks and repeat until they are all gone.
Note: The bigger your bags, the easier this is. Also, don't go too crazy with the diversity of your items. Placing 100 buy orders of 250 each is unmanageable, even if you have the maximum bag space in your inventory.
Alternate strategy: If your bank is empty, you can run your items (use the waypoint to get to the bank, then abillities to quickly sprint back to the trading post) to and from the trading post in order to get larger stacks. So if I pick up 10 gloves, 5 pants, and 14 shoulders, I can run to the bank and throw in all but the shoulders, and then pick out all the shoulders I have in the bank. This way when I list I will have a much larger stack. I don't use this method, but if you're having issues it might work for you.
Don't forget to undercut, but don't undercut yourself! Also, stay vigilant and buy out people who are undercutting by way too much, or at a price point that you can't sell for a profit.