Hi–long ago I sold a small product on the Seeed marketplace, and still build very small runs (I sell about 10-20/yr) of little SAMD21 boards, but struggle with the many pins in a home workshop and particularly struggle with USB connectors.
It occurs to me that the Xiao, with its built-in USB connector and pre-soldered pins, would make an excellent module for anything I need (basically uart access). Is it permitted to purchase the Xiao and sell a product using it as the CPU, much as I would use a base SAMD21 chip, or is that prohibited?
As long as you are purchasing these boards from Seeed Studio or a recognised and authorised dsitributor, there should be no issues with doing that. It is no difference to purchasing a Raspberry Pi and making a commercial device with it. You accept the warranty and fit for purpose yourself. Seeed probably has a clause somewhere that says any faulires or fit for purpose are down to you. You should include a small copyright in your documents that state that the copyright name Seeeduino Xiao belongs to Seeed Studio, if you need to mention the name in your product literature.