In the Appendix of Hoard of the Dragon Queen, it gives the following information about the Red Wizards.
Red Wizards are the would-be magical overlords of Faerûn.
Feared by the general public as evil tyrants, Red Wizards trade their magical inventions for slaves, steel, silk, and gold.
As magical artificers of great skill, they command high prices, and are not afraid to extend credit—for there are few who could survive an unpaid debt to these wizards.
Furthermore, in Sword Coast Adventurer’s Guide they are described as:
The zulkirs and their underlings constantly vie with one another for power and influence, and this competition frequently sends Red Wizards far from Thay to seek new spells, recover lost artifacts, and create wealth that can flow back to Thay.
Creating wealth is very broad, so would be open to whether it included selling magical items.