If you're in the market for an older trailer, your horse's safety and the trailer's roadworthiness should be your primary concerns. Unlike buying new, you won't get a warranty when buying used, so here are some critical shopping tips:
Floor: Thoroughly inspect wood floorboards for signs of rot or softness. Also check the condition and number of the steel cross members (the more cross members under the floor, the stronger it will be). Check a wood floor's frame for deterioration or rust, and inspect an aluminum floor for signs of corrosion.
Tires and wheels: Always check wheel bearings and tires for wear. Uneven tire wear can indicate improper inflation or even bent axles, so check for axle damage. Check electric brakes and pads. Wiring is often a problem in older trailers. Make sure that the tires are trailer-specific rather than automobile tires, and inspect for dry rot (small cracks all over the sidewalls from sun damage). Bring a tire gauge, and check pressure to see that it's even in all tires, and that tires are filled to the maximum psi allowed (usually indicated on the tire).
Coupler: Inspect a tag-along or gooseneck coupler for visible damage and check that it works correctly. Make note of the coupler size so that you use the right ball. Also check the gross vehicle weight rating (GVWR), which should be on a sticker somewhere on the trailer (this isn't the trailer weight, but the weight it will hold safely).
Dividers and doors: Check that interior dividers, butt bars and breast bars work easily, and inspect interior surfaces for any visible hazards. Make sure all doors latch properly and all windows work.
Vehicle compatibility: Make sure your tow vehicle is more than capable of hauling the trailer fully loaded. If you're shopping for a gooseneck, check that it clears your truck bed by at least 8 inches (the newest four-wheel-drive trucks are often too tall). Ask the seller if you can hitch the trailer to your vehicle before you buy.
Get a professional's opinion: A horse trailer is basically a box on wheels, with no engine, computer chips, spark plugs, etc., so any good body shop should be able to check a used trailer's soundness. Make an appointment for your potential purchase to be professionally checked out.