You should mention/show the serial number on the Metal Slug 6 cart. There are lots of $40 Atomiswave bootleg carts coming out of China. Metal Slug 6 bootleg carts usually have serial # 3500510.
$350 for a legit Metal Slug AW cart and motherboard is a deal, but like tomwaits stated, the market has been flooded with bootleg AW carts lately. A picture should erase any doubts. Good luck with the sale.