How to export a table with transparency from Numbers to PNG? If I copy a table from Numbers to the pasteboard and then create a new document in Preview, it usually works fine, but if cells contain partially transparent background or border colors (especially within gradients), the alpha value is either not kept or it’s more opaque than anticipated.

Maybe this is a limitation of or issue with the PDF format that Preview defaults to, or of the underlying framework. Anyhow, I already reported a bug to Apple. 

OS X 10.10.5, Preview 8.0, Numbers 3.5.3. I’m willing to upgrade if needed. 

Meanwhile, is there a way to export a table to PNG while keeping alpha information intact using just system or free third-party tools?
It works in Photoshop. You can select a transparent or translucent cell fill in Numbers. Copy the table from Numbers and paste into a new blank document in Photoshop, and then hide the Background layer in Photoshop.

Here's another angle using built-in system tools, but you might need to be a bit proficient with AppleScript:
https://iworkautomation.com/numbers/examples-table-as-image.html