Do I need to re-invent the wheel and have all the cost or is it ok to buy a pre-existing platform - like a white label classifieds app - and then customise it to fit my classified app needs?
Everyone does this.
For example, take the entire LAMP ecosystem.
You could write your own Linux + Apache + MariaDB + PHP + OpenSSL + WordPress...
Or you can just use these, as they currently exist.
Same applies to ideas.
Dan Kennedy is found of saying, "Pioneers come back riddled with arrows..."
In other words, doing anything new is likely to fail.
Better to take something... anything... already working... put your own spin on it... then go after part of an already existing market.
As for your specific project, you've provided to little detail... to provide any in depth advice...
Since your project is Tech based, likely good to bring on board someone to serve first as a Fractional CTO (Tech)... then after your tech is working... change over to a CMO (Marketing)...