I think the likelihood is high that the demand isn't nearly as great as you think it is.
I've seen restored fresh air heaters sell for $300 less than a year ago but recently they seem to have dropped considerably in value. It's been my observation that anything old goes through wild swings in price and with the uncertainty of the economy right now, everything seems to be slowing down.
I'm not familiar with the heater you were trying to sell so I can't comment on that one specifically... got pictures?
