I'm considering selling mine, and wondering what would be a good price. It's loose, no packaging, instructions, or anything. And all complete I believe(larger head robot, Cerebros, and the little satellite dish thingy). I've heard that the Maximus head robot came with a gun, but I don't know if that's true or not, mine didn't have one when I got it.
I just paid $300 + shipping for mine about a month or so ago. That was complete both Headmasters, ramps, satellite dish, and minty fresh stickers. Assuming he's complete (you said "larg headmaster" but made no mention of the smaller one) I think that's a fare price. If the smaller one is missing your guess is as good as mine, he can get rather pricey. EDIT: Oh yea also had the little gun, I tend to forget about it, I just threw it in the "jail cell" and left it there.
The larger headmaster robot - Brave - did come with a red gun. Brave Maximus did not come with any guns (except for some lucky draws), but did come with the stuff you described. From what I have seen you could try the 2-300 range for it. If you are missing Plasma (small Headmaster), then I would guess less.
Loose complete Brave Max is around $200~250. A MIB is about $350-400 and misb $500-600... that's how the prices have evolved on ebay at least.
Car Robots, original one, not the Korean "reissue". And yeah, I do have the smaller headmaster, Cerebros as he was called in RID, though my dog did get ahold of him a couple years ago when I had the big guy down to clean the dust off of. Chipped the head mode face on the side just a little bit. I know the larger headmaster robot has a name too, but for life of me I can't remember what it was. Savior or something?
I only paid $140 for mine. It was $135 plus $5 flat rate shipping from BBTS. That shows how long ago it was. BBTS had flat rate shipping. This is not a fair post as I bought him when he was new. That was his retail.