Hey might be late but if you haven't found the solution yet.  Check out backflipconsign.com -> they help you manage consignors, payouts, inventory, and works with Shopify. 

Hey,  maybe backflipconsign.com might work. 

Hey,  Check out backflipconsign.com, we're a consignment software that worked with Shopify.  We help with managing consignors, payouts, adding and updating products, and more.