Welcome to SotE.
I don't know what the shop is or why it is there. I've never been able to use it for anything so I just kinda assume it's there as a moot feature.
I don't know about a mage guild, per ce, but like Brisinger said, you can always start one!
Look forward to interacting with you and hope you enjoy it here as much as the rest of us do. :D