Everyone hates the Thornbirds, but I think they are a cool looking tire. They ride like sh!t, and you will probably have problems with them (random flats from the casing coming apart), but if they are dirt cheap on CL then why not run them for awhile? Not sure what size those are, but if they come with the wheels and they have some tread on them I still wouldn't go over $250-$300 for the set including the wheels. If the guy is asking $400-$600 then pass for sure.
I had some Thornbirds when they first came out and they looked really cool. It was on my daily driver, and I made them work for a couple of years, but was VERY glad when I got rid of them. On an occasional driver they might be OK. They are "bias belted", so they don't flat spot as bad, but the ride ain't great. I've been running Boggers on my K5 (mall crawler) and I won't do those again either, but the Tbirds actually roll a little smoother than the Boggers.