Don't let sellers rip you off! I checked the Kelley Blue Book value of a 1991 CRX HF w/ 150K miles (www.kbb.com), and here are the results, for private-party (non-dealer) values:
You're $1500 offer for a good 1991 CRX HF with about 150K miles is perfectly decent. I've made the mistake in the past of paying higher than blue book value for a car simply because it was "hard to find" (a 1996 Camry Wagon). Since then I refuse to listen to other's BS. Unless it's a classic car, do not pay more than KBB value for it!
I don't think that the Si model ever got 49mpg - in 1980 or any other year. Last I checked the Si got about 30 (stock), EPA estimates. The one in Reno claims 43-49 - so right there he's not telling everything if it's an Si.
Looking to trade for an early 1988 Honda CRX HF (Pillar mounted seat belts)