To know that a used trailer is a good price, look at the National Automobile Dealer's Association Guide, compare prices with a local trailer retailer and look at prices on eBay.com. Even if there is no plan to buy from eBay, it is a good source for average pricing.
Using the National Automobile Dealer's Association Guide is not the most reliable option, but many people who buy trailers turn to this resource first. It is easy to do online, and a value is available in minutes. When it comes to trailers, however, the National Automobile Dealer's Association Guide bases prices on statistics, and not selling prices. In most cases, the actual price is on the low end of what the National Automobile Dealer's Association Guide reports.
Looking for similar trailers from local dealerships is also a good way to know what a good price is for a used trailer. Though these prices may be higher it is still possible to get a ballpark figure on price. Using eBay is a valuable service as looking at the final selling price is a good indication of what a particular trailer sells for. To search eBay, look in the recreational vehicle section, type in the make, model and year of the trailer, and then click the box to narrow it down to completed listings.