We have seen that Magento Marketplace sales are subject to a 70/30 revenue share split. This is only a matter of the sales that are made through the Marketplace. This means that you can sell your extension by yourself and keep the entire revenue.
The best Magento product type destined to sell extensions is the downloadable product. Thanks to this product type, your extension can be sold easily.
I recommend you to lend to your customers a version history: they will be able to download the exact version they need when they install or update its website. At the same time, showing all the versions will be a useful shop window for informing your customers that the extensions are live and are maintained!