Gotta second the CL idea. I've bought numerous trailers over the years on CL, including a 20' heavy equipment hauler I use to transport my Deere 1050 and a 7k dual-axle landscape. You definitely need to know what you're looking for though, and know crap when you see it. You have to make it a point to check the listings every night or early morning though, the decent stuff will go fast. You should also know the price of stuff new, and what the repair cost of different things will be, so you can judge if something off CL is a deal or will cost you more in the long run.
A lot of people will price their trailers a few hundred $$$ under the cost of trailer new, but by the time you change tired, bearings, an axle, or other repairs depending on what it needs done, you're looking at exceeding the cost of a new trailer. I ran into one idiot who was selling a 16' dual axle with a disintegrating wood floor, 4 dry-rotted tires, and more rust than a derelict barge. When I pointed out to him that repairing all these things was easily going to cost me in excess of $1000 to fix, which, when added to his "bargain" price, would exceed the cost of me buying the trailer brand new, he feigned outrage, flexed his willie, and told me to "get out of here for insulting him". *shrug* Another time a retard took some plywood and bolted it to a boat trailer, and was selling it as a 5x12 "landscape trailer"... oh... the horror stories!
Oh, and the other thing to watch out for are the idiots who think their trailers are "heavy duty" some somehow qualify for a higher price. Granted, some are made out of 2' angle iron compared to 1.5', but the real "heavy duty" stuff will be i-beam or tubular steel construction (and is definitely worth more). When buying, consider a tubular steel construction trailer may be nice, but if it only has a 3k axle, does it really matter to you if its tubular steel or 2' angle iron for the extra money? Both will work just fine, haul the load, and give you years of service.