These are the things I look for in all of my used equipment decisions:
1. Cosmetics - Have to look new, no scratches, dents, stains or damage. Have the seller send you hi res pics with grill on and off, serial numbers and all sides of the speakers.
2. Buyer profile - Do your research. Look to make sure they have good feedback somewhere in reputable auction or HT sites (ebay, audiogon, HTF, etc.)
3. Documentation - Ask for receipts from the store they bought it from. Are manuals, warranty cards or original packaging included? Having all of the original documentation and packaging helps to indicate that the seller takes care of his stuff IMHO.
4. Hours in use/years owned - Just an indicator of how much the speakers/equipment have been used. Only a minor consideration for me to determine price or value I will pay for the equipment.
5. Pricing - Again, do some research to see where used pricing should be for that piece of equipment. Don't find another perfect specimen later for significantly less than what you paid for now. Be patient! Set a low price in mind then guage whether a current offering meets that low price or is worth more. I was able to get my two amps at 50% of MSRP by waiting for the right seller and conditions. Both were in perfect condition at a great price.