Used dishes are fine as long as they meet the following specs:
1) dish should not be warped or damaged in any way
2) most fiberglass dishes have a c band only mesh inbedded, i would steer clear of fiberglass for that reason.
3) for reliable KU band reception, if the dish is a mesh dish, make sure that a #2 pencil will not fit thru the mesh.
4) most people recommend at least a 8.5' dish for reliable c band reception. Smaller dishs could have "bleed thru" due to the close spacing (2 degrees) of satellites today
I have a used Channel Master 10' mesh (actually pierced aluminum) dish that i picked up last fall, dish is at least 12 years old but works great.