If it sounds too good to be true, it probably is. My buddy picked up one like that, Claimed to be 150amp+ and we couldn't get more than 80 out of it. (Don't think it was from this particular company, there are a few out there.)
I had Mike Singer (Singer alternators) make me a 250amp alternator for like $400ish. It tests at 255+amp.
He makes ours for like $250-600 depending on amperage. My car stays pinned at 14.9v when its running and I have over 4500 watts of amplifiers. Lots of people run his custom alternators around several car club forums.
