As the "ol' goat" (older'n me anyway) says, make sure it's the real deal.

You need to get friendly with ebay on stuff like this. It's a decent indicator of probable market value. Between the current listings and completed listings there's a history of market interest. And there are probably many multiples of web sites dedicated to these beasts that may also have for sale listings or chats that talk about values. Of course, standard for sale listings are ASKING price, not selling price. And a caveat on ebay is that not all things that list as sold are completed. Sometimes those with bid fever get buyer's remorse, but by and large ebay is pretty representative.

Just took a quick look, some examples.

There's a complete engine there. Last week it was bid up to $5800 and closed as a no sale, under reserve. This week same engine, same seller, has it up at $4700 starting with a $5600 Buy it Now!?!? Big negative on this one is that he claims that the block was "decked", so the stampings on the block pad are gone!! Could be why last week's bidders ain't bitin' no mo, once they thought about it. Yeah, it's got the right casting number, but................

Then there's some examples of parts;

Heads......................sold $1125
Crank.......................sold $475
Clone inductions......sold, $1200, & $2300 Real ones are supposedly "worth" $12,000 - $18,0000 suppose that depends on how desperate the buyer is for originality.