the main things i look for when buying a trailer are:
material used to build (a lot of trailers around here are home made). i usually stand on one corner (left rear for example) and bounce up and down. you can tell how stoutly its constructed by how much the opposite corner moves. movement is fine, just if its swaying in the wind, it might be a little light duty.
make sure the lights work.
ramps are a nice bonus. even though you can probably drive ur jeep on it without them, at some point you might have something broken and ramps are definetly nice then.
i pulled my jeep on a 16' bumper pull with no brakes for a while. it wasnt that bad (esp w a 3/4 ton). BUT....once you pull with brakes, you'll never go back. If the brakes are regular trailer axles you can add brakes on later, but its not cheap. that was why i ended up selling my 16' trailer.
check the tires not only for if theyre rotten or not but what kind they are. i prefer truck tires over trailer tires but i've seen them before with "p" tires off of a car. i've spent more time on the side of the road changing trailer tires than should be allowed....so dont skimp on the tires.
look at the springs and hangers for the tandem thing (between the 2 axles) and make sure there no broken welds etc.
heres my new trailer i picked up a couple weeks ago that will pull jeep carrying duty (among lots of other things). its currently undergoing paint and general clean up. if the price is right, you can modify the rest as needed