Look at prices of similar new trailers at suppliers and catalogs. If the used trailer is in good condition, figure 50% of new price. Start bargaining less than this figure keeping in mind your top price. (If it is a nice trailer and you really want it, you have to go a bit higher.) If some repair is needed, reduce the price accordingly.

Jack up each axle end and spin the wheels. Listen for bearing noise. It'll be a grinding like, grumbling noise. Good bearings are quiet. A common problem with trailer wheel bearings is water invasion, especially for boat trailers where the wheels get immersed during launch and take out time.

A poor bearing will likely fail on a long trip at highway speeds.

Before you finalize the deal, check out the lighting. Does the tail lights and signal lights work? Does the correct set of lamps flash when a turn signal is turned on? (I am assumomg that your tow vehicle is equipped with the proper setup to handle trailer lamps. If not, get it equipped before picking up the trailer.)

Match capacities. The capacity of the vehicle hitch should equal or exceed the trailer and the tow vehicle must be rated to handle the trailer load. Same goes for braking. Match the hitch ball size to the railer's tounge. The trailer must have safety chains. Criss cross the safety chains and pass under the trailer tonge, e.g., the left chain goes under the tongue and hooks up to the riight side of the tow bar and the right chain goes under the tongue and fastens to the left side of the hitch. The object is in case the tongue comes off the hitch, the chains will cradle the tongue keeping it from digging into the street or soil - think stuffing the tounge into a pothole. Properly installed chains will keep the trailer fastened to the tow vehicle, yet not bind in a sharp turn, and will and prevent a run-away trailer. A loose trailer on a highway is a lethal object. and you are responsible for it.