First, a used Volt is an awesome used car choice that has rock solid reliability, economical to own, and is fun to drive. The one you are looking at is loaded with every feature (short of polished wheels) available. The 20K miles are still under the 36K factory warranty. It's a great find.
Second, I'm going to offer that KBB, NADA, and the other typical guides are historically
inaccurate for cars with large tax credits (like the Volt). Contrary to what these services want you to believe, these sites mostly base their numbers off an algorithm that depreciates off of original MSRP. Their software doesn't know how to accuracy calculate the instant depreciation of a $7500+ tax credit and are, in my experience, largely worthless in predicting actual cost of a used Volt. Cursory checks of actual used Volt sales prices posted in this forum will offer proof to that claim.
Finally, in my opinion, that price is way too high. I paid $18,500 (after tax credits) for a brand new base '14 with Safety 1 back in August of 2014. I've not been looking lately, but around a year ago, a car like that was selling at auction for ~$12,000 ±1,000. I'd estimate $10K-$12K is close to what they paid at auction (or offered at trade-in). Add on dealer prep expenses and modest profit and $14K isn't unreasonable. I'd not offer a penny above $16K. As others have said, you could get pretty close to a brand new Gen 2 for not much more than the $18k that are asking for this used one.
Remember, this is a buyers market and you have the negotiating power. Gas prices are low and will be for a while longer. Most current buyers want CUV's ad SUV's and not small hatchbacks. Many used car buyers are uneducated about the benefits of electric cars and will not pay a premium for them over a lower priced ICE vehicle of the same size (Cruze, Civic, Corolla). Even more used car buyers are unsure of the reliability of the battery and electric powertrain. Don't feel you need to rush this... offer low and stay firm with your price.
Edit:
Here is a recent example of a forum member who purchased a almost identical 2014 with 36K miles for $11,000.
