Dear Ms. Schrödter,
Thank you for posting your request on History Hub!
It is the policy of the National Archives for employees not to sign documents stating that particular records are not copyrighted since government records are in the public domain. For other materials, it is the user's responsibility to identify the copyright owner and to obtain all necessary clearances before using the records for purposes in excess of "fair use."
Most government records are in the public domain, however, some of the records in NARA custody may have donor, copyright, or other restrictions. If the records have been described in the National Archives Catalog, restrictions will be noted in the individual descriptions. That would include captured German records, which seem to be what you purchased. The most likely reason you would not be able to use them as you wish is if you made a legal agreement with the seller on eBay that you would not publish or reprint the information or the records contain some copyrighted material such as non-government publications.
There are presentation requirements for some items, such as declassified documents where the declassification number must be visible on all printouts or scans.
When you do publish the records, we ask you to cite them according to General Leaflet 17, Citing Records in the National Archives of the United States. If indeed you have captured German records scans from microfilm, the citation guidance begins on page seven of that document.
We hope this is helpful. Best of luck with your research!