People buy these things, it is a big craze now. Everyone who puts $500 body kit and $1000 rims on their car to make it look pretty wants lambo doors. They just hit the market last year and Extreme Doors is among the top 3 manufactures for the kits right next to Decah and Vertical Doors who's kits wholesale at $1300
The design and components of the hinge system arn't cheap, let alone the shocks and material. Many people on cardomain advertise where they got all their parts from. Car domain doesn't care.
As far as $100 a sale, it's a pretty fair payout considering how cheap the kits are being sold for. Not lot's of profit is being made, maybe $100 for you and $100 for Extreme Doors without any recurring sales ;) Plus the kits are very good sellers so with the right amount of traffic you can expect at least a couple sales a day.