What is the size and asking price? If it's $1500 or less it's a good deal.
Like earlier mentioned, check axles, look at the tire wear, check wheel bearings, look at the wiring. Do the trailer brakes work? Hook it up and take it for a drive. Does it track behind your truck well?
The thing is with trailers is that they are all just boxes on wheel. They don't evolve unless you start running heaters, tools, lights and blingy crap. For the price of new ones you can "upgrade" the old one the way you want and still have money left over.
I recently painted mine, added a side door and am in the process of wire brushing the frame and axles and spraying a rubberized undercoating (Summer work now). The plywood is solid but can stand to be replaced. You can always put some new flooring on over the wood, or exterior carpet.