Monthly sales reports
This seems like a perfect usecase for the Forena module. Forena is built of the idea of using SQL to get data out of a database and use XHTML and CSS to format it into web reports (and/or charts). Various DBMSs are supported, such as Oracle, MS Sql, Postgress, SQLite, MySQL, any PDO compliant DBMS ... And there are quite some reasons for considering Forena.

Visit the Forena HowTos page for some links (near the bottom) to some live demo / showcase links, and experiment a bit with some of its demo samples.

Some of its features that I think will help to answer this question (and which only is a matter of minutes to deliver the features you're looking for):


Create a report about "products/services sold during last month, quantity and the price." Have a look at the "Northwind database" demo videos, which I tink are pretty close to what you're looking for. Such as these videos:


How to create a report/graph about Shipping summary (fast forward to about 4:15 if you're not interested in the SQL query prototyping part of it).
How to create Crosstab Tables and (related) charts (fast forward to about 2:15 if you're not interested in the SQL query prototyping part of it).
Various other Video demos and tutorials created in an earlier (7.x-3.x) release of Forena (which are mostly still accurate, but only using outdated Forena screens that have been enhanced in the 7.x-4.x version).

If you don't want to implement some validation about "... that the quantities for all products are set?" ... (or cannot do so for whatever reason), then you could create a report to find all cases that violate (as an alternative).
To create a report about "identify which users have entered the report and which users didn't do it?"


Disclosure: I'm a co-maintainer of Forena.