If you already have a store on another marketplace, there are many tools available which can help you to synchronise your inventory between websites. This means that you can run a store simultaneously on multiple websites, without having to manually update your inventory.
Brick Sync - http://www.bricksync.net/ - This is a free piece of software which can run on a computer to synchronise inventory.
BrickPacker - https://brickpacker.com/ - This is an online web service, with a small fee that keeps your inventory in sync.
Brick Freedom - https://brickfreedom.com/ - This is a more recent online web service, with inventory synchronisation, and some order management tools. It has a free plan and a paid plan.
Brick Seller - https://www.brickseller.net/ - This is another web-based service, with inventory as well as order management, and inventory synchronisation.