Just replaced fuel pump with used. Worked fine 1st day. Now I have to pump accelerator, will not turn on when key is turned. What can I check?

First, why did you change the pump? What was going on before? What do you mean will not turn on when key is turned? Do you mean the fuel pump doesn't run with key in ON or the engine doesn't start with key in START?

First thing check fuel pressure and check for spark.

Pumping the accelerator wont help you start it unless you are just trying to keep it running after it starts. Why would you replace the pump with a used pump? Its probably bad and you just wasted the time replacing it. Save yourself the trouble and just buy a new one.