I have an opportunity to buy one, but know very little about them. The Serial Number begins with CST. Anyone know of a place I can find an estimated value? I would rate it as good condition. It fires and cycles fine but does have some minor cosmetic dings.

I paid $1200 for a new one about five years ago. It is a very accurate and reliable rifle. Not sure of current used value, but would guess $900 ish.
Seems to me the rush to buy ARs at any price is over for now.

Is it the HBAR, 1 in 7 twist, 20 inch barrel with the carry handle sights?

If so, I had one and it was worth around $1000 bucks. Mine was a very accurate gun. Unfortunately, these days, a high quality Colt, made for High Power competition, isn't worth much more than the parts and will likely wholesale for less than $1000. Check Gun Broker.