It could be a Mauser receiver, probably 1917, but that receiver FP and serial are not Mauser style for this range. The RR suggests Mauser/1917 but not a-block and the serial fonts and suffix do not compare well. I think this is very possibly a complete fabrication, best guess is that the receiver is a Mauser/1917 from the q-r block with scrubbed LR re-FP and reserialed by a humper (which is fairly common on eGun)
I asked for the RR because the RR seems to C/R C/Dx2, and the style is consistent with Mauser in 1917 (this pattern is only known in the q-r blocks though); the FP is not real, has similarity to Danzig, but certainly not Mauser. The serial and suffix do not compare well with Mauser a-block.
Unfortunately the only way to be sure is to see the RR in detail (close up and clear) and to confirm the BC which is distinctive for Mauser (no one used Mauser's patterns).
Anyway, if eGun is any representation of Germany and Central Europe's market, then Germany, Austria and Poland are as full of crooks as the United States...