I know I have seen it mentioned in other discussions on the subject of pricing that for used equipment, a "ballpark" figure is about 50% of new cost - depending on condition,age etc the actual price may end up higher or lower.
Around here, used plows go pretty cheap. I've seen them advertised for anywhere from $100 - $800 for 7.5' and 8' Usually brand isn't specified and you can count on them being rusted. I'd say, being in good condition, it should be worth half of new. You will want to specify what year and model of vehicle it came off and whether you're selling it complete, minus headgear etc.
If your looking to get the most for your plow I would wait until around the middle of August and see what a new plow is selling for "not installed" and then list it in the Advertiser or the Trading Times. I would keep asking if anyone knows someone looking for a plow through out the summer.
I would start at a third of what a new plow is if the plow is only a few years old and in "excellent condition", and go down from there. Remember that the price you paid new may have been with installation, and you should deduct that from what you price your plow at. Be sure to know what "other years" of trucks that "your" mount and harness will fit. No one really wants to pay top dollar for a used plow and have to lay out more money to make it fit and operate on another brand or year truck.
Have an idea of what another harness and or mount cost just in case you do find someone with a differant brand or year truck who would buy it if it would fit their truck. You could always split the differance and get the sale, and sell off your old mount and harness.
Don't forget to list the upgrades that you made to your plow as extra's to sweeten the deal.
During the spring and summer is when I try to pick up most of the used plowing equipment that I plan to "resell" during the fall, it's when some are getting out of the plowing business or don't want to store extra equipment over the summer, or have a new truck coming and relize that the plow on the truck didn't add anything to their trade in. (e-mail me and I'll add your plow to my list of used plow equipment that's available.)