Because "too fast" is a matter of opinion. His/her experience/opinion may be that items that take 3 months to sell are sold at a price that "they are worth" - because by then the market has had the opportunity to value the items at it's maximum (read: sold at the highest possible price for the seller), even if, in most other peoples opinion, they are asking "too much".
+1, that was my understanding of what he meant by too fast. First listing. There was a guy o'er in Oz selling a vanilla A2000 for $499 listed for over 18 months ago, he's stopped listing it now, so I'm guessing it sold. Time isn't important for these people, it's the price you can get for product. Personally I'd care more about the home the unit was going to and the fact that it would be looked after. I'd get more joy giving away one of my A500's to someone who would give it a new lease on life rather than make $ on it. But then again I don't need the money or the income supplementation. Other people do this as a business.
The market rules the price. Aside: The Scotts and Yankees started this whole business off in the 1700s with tobacco.
@psxphill: The A4000 went for $790. We might think that's ludicrous. I kinda do, but for a fastidious collector it might have had all the right elements. Obviously at least 2 of them thought it did. Either that or there was a phantom in there.
Also I don't think you compare a Indvision with an a2320. One is vintage the other new,ish.
Having said that you can get this one for $160 au:
http://www.ebay.com.au/itm/Amiga-2000-2320-Amber-card-P-/200905030438?pt=AU_Computers_Vintage&hash=item2ec6df7b26
