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