Have you tried logging off and logging back in? That usually works for me to get mail.
Is your mail inbox full?
Are you certain that you had items for sale at the time of leaving that specific trade guild? Maybe try finding their guild trader in game and searching for any items you might have had for sale. See if your name is attached to anything.
Are you sure they were in the TRADER and not the BANK?
If the stuff was in the TRADER, it is mailed to you immediately after you leave. At least that is how it is supposed to be.
If it is in the BANK, then it will remain in the guild.
I know it sounds obvious but just trying to understand what happened because you should have received it. I've left many trading guilds before and never had issued getting my stuff back.