Import/export is the intended feature for this use case.
1. Select the desired collection(s)
2. Select Export from the gear menu
3. Select "Selected collections" in the Export dropdown
4. Select "JSON" in the As dropdown
5. Click "Select destination" button and save the file
6. Import the saved file into another SB installation via the Import action in the gear menu
v4.1 (due out soon) will include a Copy button in the Export dialog as an alternative to saving a file.
Fair point about this not being accessible through the collection tile menu. It should probably be added there.
Copy/paste is for session or collection content (tabs or links). This feature allows copying a selection of links across collections (and even the current session). Due to security, it does not work across browsers by default, but you can enable that by giving SB system clipboard access:
1. Right-click SB icon
2. Click "View web permissions"
3. Select "Allow" for Clipboard
4. Reload SB
This must be done on both the source and target browsers.
Let me know if you have any other questions!
Cheers,
Hans