SQL Query with SUM with Group By

The GROUP BY clause forms rows for each unique combinaton of the columns you nominate in that clause.

If you want to show a sum for each month, then only include s.month in the group by clause: e.g

SELECT s.month, SUM(rs.total_reseller_sales)
FROM sales_report_resellers rs 
INNER JOIN resellers r ON rs.resellerid = r.resellerid 
INNER JOIN sales_report s ON rs.sid = s.sid 
WHERE (rs.sid > '294' AND rs.sid < '306') AND r.insidesales = 0 
  AND r.resellerid IN (7, 18, 22) 
GROUP BY s.month

If you include reseller details in the select clause also include them in the group by clause then there will be one row per reseller AND month