How much should I expect to pay for a used boat trailer?

If the trailer is pristine, $500 is good. If it has been used in salt water, forget it. Does the trailer have bungs or rollers? Bring a small jack and see how the wheels spin without a load on them, listen to the bearings. Check the electrical system, (just make sure it is all there). If it is an average trailer, $300 is a good price for you, $400 is a good price for him.