There are legitimate services commonly used to buy stuff from US sellers who don't want to deal with the complicated shipping to Russia and other countries. I doubt he had any intention to obscure his identity – the seller would just not sell to him directly.
It's only legitimate so long as it isn't used to circumvent ITAR. The seller would probably not sell to him directly because exporting flight manuals for military aircraft was paper work the seller didn't want to deal with. By obscuring his identity through a third party the buyer violated ITAR, the seller probably also did as well by not doing proper due diligence.
Is there any suggestion that there was anything to learn from manuals being sold in eBay that wouldn't already be well known? Did the person selling the manuals get prosecuted? Was someone court martialed for releasing them?
the reaction to the crime may have lacked proportionality but there are more details to the story.