Is it the first basic 4" lift or X series kit?
Have a look at their site:
See which one it is.
If it has the adjustable control arms with Johnnie joints on one end, then by all means, grab it!
Heck, even if not. You can upgrade to the newer style of control arms later on.
Which shocks does it have with it? The standard shocks are a little rough. I run their 2+2 shocks, and I drove my Jeep clear across the country on a month long road trip last year. Great shocks.
I started with the 1st generation kit with adjustable control arms and I was very pleased with it for quite some time.
It flexed like a champ and the springs are still as good as the day I put them on. Eventually the control arms gave out and I upgraded to the X series. One day I'm going to add JJs to the other end of the control arms.
I am a real fan of Rough Country. Not only because I have had good luck wiht their products, but because when I did have small problems, they took really good care of me as a customer. Buying used, you won't have the luxury of warranty but you'll still get their good service if you need it.
If at all possible can you post up a picture of the parts? If not, a good description of everything included - MAKE SURE YOU GET ALL OF THE BOLTS TOO!!!
We can go from there, show you what to look at for trouble spots to be sure the kit is worth buying.