Not invented here!

This refers to the principle that, when faced with a choice between building or buying something, we should err on the side of buying.

Building something yourself is almost always more expensive than buying it. Not only do you pay for the initial development costs in the short term, but also the long term costs of maintenance and support.

A good principle if to limit custom development to core domain services – the services that are unique to your business and provide a competitive advantage.