How do I sell a WordPress plugin with automatic updates?

1. Install and activate Sell License from Plugins > Add New.
2. Create your plugin under License > Software.
3. Create Software License products in Products & Pricing. In each product's License Settings, under License Style, tick Bulk license: one key for several sites and set Sites Allowed (1, 5, unlimited or a number). Then tick the products on the software's Pricing tab.
4. Tick I sell WordPress plugin licenses under License > Licensing Manager > Help, then open License > WordPress Client, choose the plugin and click Download Client File. Add it to your plugin as the file explains.
5. To release an update, add a version with the plugin zip on the software's Versions tab. Licensed sites see Update now in wp-admin.

Staging and local sites (localhost, *.local, *.test, staging.*, dev.* and the usual host staging addresses) do not count toward the limit when Staging and local sites don't count is ticked.

The full guide: Sell a WordPress plugin with Sell License.