Take Amazon store as an example:
You can link your Amazon store after your Circle.us account is registered. Go to Orders>My Orders>Create Order or Settings>Selling Channels, and then click Start to Connect.
Select "Amazon", and the following screen will pop up:
Please follow the instruction and fill in your "Seller ID" and choose "Marketplace Country", add MWS Auth Token and store name, You will be all set then!
Be advised that Amazon only opens API connectivity to sellers on "Professional" selling plan (also known as "Pro Merchant"), which costs $39.99/month.