The best way to set the value of a used horse trailer is to compare current prices on auction sites and classifieds sites. You can also look up current going prices in guides such as the Horse Trailer Blue Book.
One of the biggest mistakes you can make when setting the value of your used horse trailer is to be overly emotional about the process. While you might feel strongly about a particular trailer due to your memories of the horse or horses that use it, most buyers only care about the actual condition of the trailer. By being honest about the current state of the trailer and any damage to it you can set a price that is competitive compared to other listings.
Online auction sites such as eBay.com are a great resource to check the current selling prices of different models of horse trailers in different conditions. For example, if your trailer has a structural or appearance issue and you're not sure how it might affect the price, look on eBay to see how other sellers have adjusted the price in similar situations. You can look at listings that have sold to ensure that the price adjustments resulted in a successful sale.