as long as they include a schematic for their products, i'm set

i've always viewed m5stack / seeed / tindie as 'dev board marketplaces'; once i proof the rest of the design using their board, i take the known-working schematic and simply cut out the parts i dont want to spin rev 2, then once that is working i optimize the remaining parts for rev3, and project done :)

The better phrase is probably “reference design” marketplace. When I worked as an EE pretty much all but the hardest boards (which needed to be simulated) were glorified stripped down reference designs.