Does anyone know any thing about Gross market value accounting for an internet marketplace platform?

I have a client that is a marketplace platform for a robotic delivery service.  He is wanting to do gross market value accounting and I need the best (and easiest) program to get him started on this.  We don't want it to show for external accounting, but his internal accounting is very important to him.  He wants to track what each vendor is selling thru his platform as his fees are based on that total amount.

Is there any thing I can use to do this for him?  I prefer not to set up an excel spreadsheet for this, as eventually it will crash.

I appreciate any information.