Every shock I have purchased used has never been what the seller claimed they were set up for. They swore up and down that they were set up correctly, but sadly they were not.
Does the compressed length of the shock match the length when you quad is sitting on 2x4 minus shocks?
Does the extended length of the shock match the extendend length of frame?
Is the valving/spring/crossovers set up for your weight, riding style (trails/mx) and skill level?
Without making sure those are correct, you will never take full advantage of the suspension. I have run "used" suspension and it was ok, but once I send the shocks out for the correct stuff I mentioned they were awesome. I may need to change compression a click or two or rebound, depending on the track but overall the shocks work like I thought they should.