I'm looking for an opinion from stock pros - if this kind of license can really exist out there.

I discovered one of my photos being sold by a US-based photographer through his own website as print as well as digital download. Needless to say, he never mentions not being that photo's author. He must have licensed my photo through some agency, otherwise I don't know how he could have gotten a high-res file. But is there really a license at any agency, which allows reselling of digital files? I mean that would basically undermine any agency's business - anyone could license an asset and resell it for whatever price they want.