this is something thats not confined to just the Amiga. Pretty much all retro gaming markets have people buying cheap to try and sell off as "super rare, extra vintage, hard to come by" items. Just Amiga stuff tends to be sold for crazier prices 
I think it also depends on how famous the brand is or console model is.. You can buy pretty cheap Atari / Nintendo consoles... but of course Commdore Amiga just has that awe surrounding just the name of the brand.