Sadly, some people buy old Amiga stuff just to sell on Ebay. Then they do a quick search on the web to ascertain the part's value, find Doomy's site, and believe his inflated value claims. Then they try to sell it for this price on Ebay (and some buyers believe it). The Ebay auction for the early A2000 keyboard last month(?) even linked to Doomy's site as "proof" of the $1200 value.
He seems to think that if he repeats his beliefs about a certain piece of hardware (A2000 being the flagship model / military spec), or a price ($1200 keyboard), enough times it will eventually become fact. In some ways it has, because of other sellers using his advice as a pricing guide.
Have you seen the prices he assigns his joystick collection? I would say they are inflated.