Simple search GUI has text inputs to enter some search text, lists to select from, radio buttons and check boxes - some combination of these widgets. The user enters, selects and checks some of these and click a button and wait to get a result in table or a list or just a number as a message.
When the user requests for a result, the inputs from the user are sent to the back-end / application where the inputs are transformed into a MongoDB query. The query is further sent to the database server, where it is executed and the result is sent back to the application and then the user interface.
This is your application. This can be a desktop application like the Compass or a web application.
I agree that such an application will be easy to use - as it is totally customized for your needs.
Now, this application has two main requirements - (1) the data being queried (the data in your MongoDB database) and (2) the queries which you want for your sales team. And, you have the data and the requirements.
You can possibly build such an application in-house in a few days and start using it. And you will also have an opportunity to improve and enhance it as it is being used.