SoBhere's some insight: Are you commuting or considering a career as a homeless person with a bicycle? Those are all big trailers, designed for carrying lots of gear on a cross-country bike tour. I can't imagine using one to commute with, to be honest. I should think you could get your work gear, clean clothes, and packed lunch into a couple of waterproof panniers (try the Ortlieb Mini Trekker for $175) and a luggage rack.
Price-wise, the three trailers look like this: the B.O.B. Yak Plus is $289, the Big Tow is $270, and the Nomad is $300. The Yak and Big Tow are both single-wheel designs. Advantages are easier tracking on rough terrain and the fact that the wheel layout isn't wider than your bike, a problem with two-wheel trailers. On the downside, they make for some weird bike handling, although you get used to it. The Nomad is a two-wheel model. More stable, but there's also more stuff to go wrongyou might try to aim the bike through a gap in the curb and forget you've got a wider wheelbase for the trailer.
Speaking from experience, I use and like the B.O.B. (aka, "Beast of Burden") Yak. The Big Tow is nearly identical. I'd rather my trailer take up no more road space than I do, so I'm not particularly keen on the two-wheel designs like the Nomad, at least for cargo. As kid-haulers, that style is far superior because of its greater stability.
As for used trailers, I found Yaks on eBay, but no Nomads or Big Tows. Seeing as you're near Seattle, you could advertise in the Cascade Bicycle Club newsletter (of course, you'll also have to join) and look for one that way. Good luck!