There is not currently an export format that has only isbn & no "generic" export. There is a feature request for CSV & similar formats that would allow easier manipulation (search the forums for info on this).

It would be straight forward to write an export format of ISBN only. However, this seems like a edge case that might be better waiting for a more generic text exporter (such as the CSV format).

To immediately address your need, you can process the text file produced by some other export format that has ISBN included. Are you familiar with grep or a text editor that could help you extract all lines in a file that matched a pattern?

Thanks, noksagt - I use Notepad++ for my own.

And do you know how to use this to make the requested file for your own needs?

I work in a public library that doesn't allow usb keys, and am trying to find a way for patrons to build/transfer bibliographic files.

I'm not sure how this fits into your original request. Do you expect the patrons to use google books for this transfer process somehow? If so, will the patrons not need to export articles & books that aren't in google's database?

Do you want patrons to be able to export the zotero data for use on other systems? Or just a formatted bibliography?

Presumably there is network access. Can they put/retrieve files on a server? Or just use email?