Looking at an 02 SC430
noticed a small amount of rust on the corner of the trunk (very close to the tail-light) - common problem??

 

And by the tail light I mean the point of the trunk (at the top of the tail-light)

 

Yup, I have the exact same thing...and I've seen it also on a fellow CL member's 2002 SC430.

 

worried about it spreading! is this a deal killer?

 

My '02 has it too. It's from rubbing when the top goes up/down... every year I sand the spots carefully, dab w/primer, touch up paint, and clear-coat. It hasn't spread at all; just wears on the same spot. There's actually a couple spots around the tail-light area where it happens on mine; the spot you saw is the largest, and there's a couple much smaller ones if you open the trunk and look around the edges of the tail-light area. You can also take off the upper reflector (the part of the tail-light that's connected to the trunk) with two 10mm screws and get at the spots easier.
Unless they're real bad, I wouldn't let it sway your purchase. You just have to take care of those areas with a small amount of maintenance and not worry about it.

 

Rust? I thought the body of the SC was made of aluminum?