It is an oiless air compressor. We mount them on risers all the time. I am sure Phil and Rusty are right about the whole assembly. The compressor will develop high enough pressure to run tools, the problem is volume. The tank holds high pressure, which is regulated down. Is the brass thing a pressure regulater?
If you add a huge tank it could be another air compressor.
Your price seems good, advertise it as an air compressor with extras you could get some hits or more interest.
Chilly