Couchbase supports INNER JOIN and LEFT OUTER JOINS.
You need to have reference from one bucket(document) to DOCUMENT KEY of the other one.
In your case, try the following:
SELECT * FROM sell s cINNER JOIN consumer c ON KEYS “_id::” || tostring(s.id);
Note: This implicitly means the following. But, syntax is NOT supported.
Standard syntax will be in the next release. Look for developer preview later this month.