We are buying used XPS and some pieces are heavier than others. Although I've read that XPS is not supposed to take on water it seems that this foam has indeed taken some on. Any opinions on this? Should we buy it and try to let it dry or do you think this stuff is degraded and that is why it has taken on water?

I had the same problem but bought enough that I could separate the 10 percent or so that had moisture content. (They were stored poorly.) I weighed them with a handheld scale, the kind with a hook on each end. Happily, they dried over a winter in my garage. Moisture degrades insulative qualities so I would not put them under the slab wet. But you shouldn't have to mess with it. Wet boards have a solider sounding thonk when you rap them. If you can't pick and choose, bring a scale and take a sample of the used boards by comparing them to the weight of a new board.

Even with the additional water they retain most of their R-value, and if used in an assembly that can dry at reasonable rates in both directions it would eventually dry out. All XPS will take on water if you leave it in the rain or soak it in a pond, despite being waterproof to shorter-term exposures, but it can dry too. (EPS initially takes on water faster, but gives it up faster too, since most of the water is in the intersitial places between beads, not in the closed cell structure of the foam itself.)

Sunlight has a more deleterious effect on product life than pond-soaking. Pieces that look faded with a chalky surface might be set aside and used last, since it'll be a bit more brittle than the stuff that hasn't seen much UV light.