There's only a few things that can go wrong with a trailer. First was it used in salt water? Is the frame straight? Check the frame for sever rust damage, tap on the frame with a hammer, Poke it with a screwdriver.
Do the lights work? If not it's faster and easier to replace the lights and wiring. Go LED.
Check the coupling, Is it a newer style or an older style. The newer ones can be rebuilt all couplings can be replaced.
How do the tires look, good tread, even wear? check the tire date code if they're more then 5 years old they need to be replaced.
Jack the trailer up and spin the wheels, Do they spin freely? make any noise? Grab the wheel and push/pull, is there any excessive play? Pull the hubs and inspect the bearings, races and seals. Inspect the spindles for damage. Inspect the axle for damage/bent. Some trailer have a pre-cambered axles, those will have an upward bend to the middle of the axle. Check the springs, U-bolts, tie plates, shackles, and any other bolts and nuts.
Check the bunks, rollers, keel rollers, bow stop and winch. Stand behind the trailer and have a friend tow the trailer straight away, does it track straight?
I bought my trailer for $75.00 and put $150.00 more into it.
Check out the web site "Texas kayak fisherman" these guys have done just about everything in the way of trailers for kayaks