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)...

Yes, it is absolutely fine to use pre-existing services as long as it matches the list of your requirements and you make sure that the service doesn't violate any of the terms and conditions.
The reason this is acceptable is because they are already built and the structure of the service is already established. The only thing you need to do is adapt your use case to fit your business needs.

For classified, there are many wordpress themes and PHP scripts available market that can be used readily. For more information we can discuss this over a call - so I can give you maximum value for your money. Take a look at the great reviews I’ve received: https://clarity.fm/ripul.chhabra

100% absolutely!

Hi
Unless you're doing something really unique (and even then...) it is actually advised that you start with something that already exists in order to first test the market and see that your idea/business is indeed feasible. Once you've validated your idea, you can spend money (if need be) on creating something tailor made.
By testing a simple platform first, you will learn a lot about the market, the users, what things you should do different etc....
I am a mentor on Startups.com, a lecturer, and a startup lawyer. I've successfully helped over 400 entrepreneurs, startups and businesses and I would be happy to help you. After scheduling a call, please send me some background information and the 2 main questions you want answered so that I can prepare in advance (to give you maximum value for your money). My reviews: https://clarity.fm/assafben-david

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.