Personally, I'd buy it for that price. However, any prospective purchaser should check out the threads dealing with the E-Zip for more information. At $150, the E-Zip is a good purchase. Yes, the bike is heavy and has certain eccentricities (check out the threads), but one doesn't buy an E-Zip for sheer speed.
I put a Currie conversion kit on my 21-speed mountain bike last year. With aggressive knobby tires that increase rolling resistance and reduces speed and using the standard SLA battery, my top speed is 17 MPH. With different tires that have less rolling resistance, the speed could easily be increased by 2 MPH. The area where the Currie motors shine is torque. I live in the far northern US Rockies and the Currie motor is awesome--handling hills with significant grades with ease. In my opinion, you can't beat the bang for the buck buying an IZip or Ezip, especially the less expensive, non-hub models.
what do you mean rolling resistance? Does it have some thing to do with the tire or the wheel parts?
Some thread patterns can create resistance and reduce speed. That's why racers use slicks. I ride a lot of unimproved gravel forest roads so I have aggressive (meaning big) knobbies on my tires. These slow down the bike a bit because the knobbies create rolling resistance; however, they have much better traction on the roads I ride. I'm willing to give up some speed for additional safety riding off road in the forest. Go to a website that sells bike tires and look at the various types of tires available and you'll soon learn what tread patterns have less rolling resistance (they're generally the ones advertised for commuters and those who travel paved roads).
The problem with buying a used Ezip for any price is "what are you getting for that price?" I know of people who bought used ezips and thought they got a real steal. But the bike needed new batteries or a controller or something. In the end it cost them as much as a new ezip (I paid $240 at Toys R Us last summer). The other thing to consider is the loose of the warranty. Currie Ezip/Izip have a great 6 month warranty and a used bike will likely not have this.