Hi Jim,
All the boilers and engines you mention are good quality items so they should bring in a fair price. Of course much depends on the condition they are in.
That being said, the steam engine market, especially for out of production items, can be erratic, since there is no constant supply and the knowledgeable buyers are also rather few. The same model can sell for widely different prices.
As an example, a Proteus engine without pump and boiler sold recently for 268 Euro:
As for the best place to sell, in my experience ebay is better than forums due to a much wider audience. Of course selling on ebay incurs additional costs while forum sales do not, so offering first on forums makes sense. Then move everything that didn't sell after a reasonable time to ebay.