This app counts down 48 minutes from pressing a button, which is the same length of time as a day in GTA V, which is how often you can sell a car to make some quick money.
It provides a notification, if enabled, when the time is up, and counts how many cars you have sold whilst using the countdown.
There is also a list of the car prices, but this may not be complete. Feel free to drop me an email if anything is missing.
Please note this is an unofficial app and is not associated in anyway with Rockstar or Take-Two, or any other entity for that matter.
If you are experiencing any issues with the app, please go to the Google+ support community where I will be able to help you: