One thing I found useful was to ask the guy on the phone about specific things like:
What condition is the plastic in?
Condition of chain and sprockets?
Condition of tires?
Date of last top end rebuild?
etc. . .
If you get there to look at the bike and he's been honest with you about the plastic and the other obvious stuff, then he's probably being honest about the top end, clutch, etc. . .
I had one guy be only partly honest on the plastic and stuff, and I felt like I couldn't trust him.
The guy I bought off of told me exactly where the crack was in the right number plate, and that a rear wheel bearing was getting loose.
When he told me he had just changed the piston and ring, I believed him.
Search on google or yahoo for "How to buy a used bike" articles. They'll give you lots of pointers. There are lots of posts on here as well.
BTW, I found great deals for my bro and I. 2002 RM 125 for $1500.00 and 2000 KX250 for $1500.00.
That might be a bit much to expect, but you can find really good deals if you look long enough. I spent about a month finding the 2 bikes.