If you plan to build a platform, try to find out what the most valuable transaction on that platform would be. That could be to match an employer with a potential employee. If you cannot deliver this service without the online platform, you will likely not be able to build a platform around this service. If you are able to deliver the service ‘manually’, you will need to find out whether this transaction can be standardized enough and thus amplified by using a platform.
A great resource you cannot ignore when building a platform is the following book by Parker et al.: