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?
Please please please, use what's already available.
I'll give you an example, we use https://spark.laravel.com for billing, https://signaturegenerator.co to generate signatures, https://solidframework.net to convert documents, and probably a dozen other software solutions to create ours.
There's no where that says you need to use 100% proprietary code. Usually, this is done when you can't find anything opensource to tweak or if what's available is too expensive for your budget.
If there's an option, 9/10 startups stand on the shoulders of giants by using what's available. This is especially true if the market hasn't been validated in any meaningful way.
