Hard to say.. post a pic of it, or post the link to the craigslist page if there is a pic of it, may help to see the bike.. if its in good shape the price doesnt seem too bad to me.
I got mine off craigslist about a month ago, an 03 RM125 for $2k, it certainly wasnt perfect, but I didnt need something brand new either.. Ive spent more than a few hours going through the bike making sure it is all there and in good shape, and spent alot of money now on things I didnt expect to have to buy(in new parts), maybe I am just picky?
I thought it was a perfect used bike from spinning the tires and riding it up and down the street a few times etc. I like it and dont mind doing the work on it to make it better, or spending a few dollars on it, but if I had it to do over again I may have looked at something else..
If you really like it, go see it, look for paint wear on the frame from boots, spin the tires to see if the rims and brake rotors are all bent up, lift and shake the forks/front tire to see if the headset is tight, check the sprockets for wear, take it for a spin if you can etc.. know what your getting Id say is the best way I can put it, if the bike needs parts or work, use it to bargain with the seller to drop his price.