Great price, especially if the trailer is included. Shouldn't be hard to flip a profit if you sell individually. In my area, each ski should fetch a minimum of $600-$1000 and the trailer $150-400. 650 is fun to ride, as long as you aren't spoiled by expecting better.
Buy it if you want a 650sx. Buy it if you want to risk trying to flip for a profit. Don't buy if you want something else. If it's not what you want, it's a waste of money at any price.
Personally, if I didn't own a ski, and those are both in good running condition, I'd buy them, sell one, sell the trailer, set aside the money to buy a different ski once bored. Biggest problem with an old ski is the maintenance can rape you.
I'm pulling apart my X2 to rebuild my SX. It's cheap and fast. I can't find a SXR at the right price. I'm tired of the X2. I enjoyed my SX when it ran, and now it will have a 750 with plenty of bolt ons. I don't agree with all the hate the SX gets. The newer stuff may be better, but the SX is still a good time.