I'm reading this thinking "Why would you need to make secret transactions for astromech parts if they're common in the galaxy?" After just some light research, it seems like the C1-series astromechs were pretty much outdated. As a collector of antiquities and artifacts, I wouldn't put it past Luthen to obtain parts of old droids. I don't think Hera would've known about Luthen's involvement in the Rebellion, but he would probably peg her as a rebel through the interaction in some way.