I agree with the 'good used' value of 60% of new cost, less the more condition declines.
In 2011, I gave $1600 for a 5 ton, 20' deck over, that needed one board replaced. Included the receiver, due to it having a pintle ring hitch, seller agreed to include... bought in the Pittsburgh PA area, sold due to that state no longer allowing Mobile home style ring wheels/hubs. The axles are Dexter brand, with brakes, not just cheap MH one time use crap. Reasonable price for condition and age.
If your trailer is in really nice shape, and you're not pushed for time, it's not gonna hurt a thing to price it high, and wait a bit... maybe include OBO, 'or best offer' in your ad... you just might find the perfect new owner who has enough money to keep you from squealing too much when you sell it...
Good luck with sale... Craigslist, local trade papers, etc.