You'd be hard pressed to sell the keyboard $10 more than retail. Especially when they are in stock at EK.
Sure, EK sells the keyboard for $356. But that doesn't include shipping price. On top of that, it doesn't include shipping internationally, which is not cheap. The seller has absorbed all of that cost, and is also absorbing the shipping to the next buyer, as well as PayPal fees. Not to mention, a free red Escape key, which is about $5 on it's own.