You could use XML, which in a sense can be treated or thought of as a "database", as it can also contain hierarchical structure. There are complexities with XML, but it is an option, albeit perhaps a bit contrived.
If it is just a matter of single transactions, however, a text file (or several) seems more direct.
The file system itself could probably (being unfamiliar with pitfalls using SD cards on the Netduino) be used, treating files in a folder as the transactional records themselves. You could keep all necessary information inside the file itself, even history of attempts, etc. This way files could be removed as they are successfully submitted to the actual database.