I've been fairly happy with my Harbor Freight folding trailer.
Used trailers around me are ridiculous, there was this same trailer for sale locally, rusty, faded, beat up wood deck, for $450. I grabbed this one one sale, and with a 25% off coupon. 12" wheel model. I think with the spare added in I ended up around $270-$280 out the door.
In the end though, I didn't want it folding, so I bolted and trued everything, welded it all, repainted the trailer, added the tongue box, deck, tie down loops,, flipped to spring-under-axle to lower it, notched the hangers, raised the fenders, etc etc, and ended up with a slightly better trailer with way too much time invested in it. Then I killed/parted out my dirt bike about 2 months after getting the trailer together, so it's sat in the yard holding the snow plow all this year. Hope to have a new bike for next year though. Still want to add a 10-12" rail around 3 sides.
If I had to do it all over again I'd either buy a new trailer the way I want it, or build one from scratch. It kept looking cost prohibitive, but if I tally up what's now in this one it comes out much closer.