Another user in the dark web - a hidden part of the internet where guns, drugs and child abuse images are traded - alerted police, a spokesperson said.
The boy's body was subsequently found in the cellar of a house in Herne, North Rhine-Westphalia.
Police say the 19-year-old suspect may be armed.
Searches are being carried out in towns near Herne, which lies in the Ruhr area, a heavily industrialised region.
Reports say the suspect, who is on the run, is a martial arts enthusiast.
