Issue 1: Condition of Used Products
Either audition and inspect the product before purchase or buy from a trusted dealer like The Music Room or a dealer that sells the brand where they accurately describe condition and have return policies.
Issue 2: Depreciation
Yes, speakers depreciate 50% out of the box. Many respected brands seem to be selling for 35-40% new. Do your research on this site and the other sites to determine a bid. Prices will be at least 10% higher if you use a respected dealer rather than a private sale.
I had much success purchasing a used pair of speakers in your range.