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That looks to be a heck of a price for that trailer. I doubt you'll find much lower.
Well you're limiting yourself with your requirements of 5x10, built in sides and tandem axle(7klb). You're not divulging what you haul which is fine, but 1 or 2 loaded 2500lb pallets screams flatbed to me. There are several larger flatbed trailers on CL right now that will haul what you need , ability to forklift the pallets and it's easy to build wood sides if you need side boards. Seen a couple that could be had for under $2k easy. Agree not a lot of trailers right now. Need to keep your eye out. Deals pop up. Few years ago bought a 7x16 tandem, almost new with painted 4' wood sideboards well constructed for $1500. 2 years ago, found a like new 28' enclosed Charmac snowmobile trailer, literally like new, decked out for $5k. But you're not likely to find those kind of deals every day. Knew what I wanted and was ready to buy when one came available. If I was doing any sort of trailer hauling for hire like you are, I'd be thinking flatbed/carhauler or deck over trailer with stake pockets to put sides on if you need them.
Im guessing the requirements stem from the fact that the pallet load is likely something like paver block or small retaining wall blocks. Something with sides helps keep contained with no chance of spillage. I would say that you need a barn doo style rear gate not a tilt up like the one in the add you linkd. Gona be very hard to push or slide a second pallet onto the trailer with that and leaving it off negates any benifits of having built in sides.
Bulldog trailers- great quality. Down near Chehalis