dinan is over priced. as far as i know dinan just purchases their exhausts from another manufacturer....they are still great quality but not worth the price.
for slightly more than 500 you could get a nice brand new cat back from uuc, supersprint, remus, ect.
there is nothing wrong with buying a used exhaust as long as it is in good shape...take a look for rust, dents, or any other visual damage. when i sold my uuc exhuast that i had on for approx 30,000kms the only thing that needed to be repaired was one of the welds on the exhaust hangers.