Similar looking to one I have. Reviews seem to be okay. Should be okay depending on what you plan on using it for.
I'm trying to figure out how it makes more CFM at a lower pressure (40psi) than at the higher pressure (90psi)
The cfm is a measure of the pumps efficiency. for a given pump it will flow more air at lower psi as it doesn't have to maintain pressure as well as move air. You can almost think of it like electrocity with cfm being current, pressure being voltage and the tool providing the resistance.
One of the first questions to ask is what is it being used for. Under 10cfm will cause problems on high cfm tools like sanders and grinders. My way around a low cfm compressor was to pipe it into a large reciever so that I could get enough time to paint a set of panels before the compressor chimed in and messed with the airflow at the gun.
And remember that receiver is a bomb so secondhand buyers beware. Buy new or have the receiver inspected as pressure vessels cause a lot of deaths every year.
I should really go introduce myself in the noobs section seeing as I've started posting.