I've made a plone 4 dexterity product but I don't know how to install it on my webserver. I can install any other package using buildout -- however my addon is not posted on PyPI or a similar package index.
What is the best way to have this offline product working on a webserver? Should I use the same way as in the development mode?
If I understood your question, you have an "egg" not hosted on Pypi or other compatible site.
The way we use is to put eggs in a local directory inside the buildout and add this directory to your find-links buildout section.
You can see how this works from the RedTurtle buildout here: https://github.com/RedTurtle/deployments.buildout.plone/blob/dc2808a0130d1132365b2a1216f8ef0742a96b2b/config/base.cfg#L14