Don't know about where you're from, but over here, $350 is a very fair price considering I've seen local sellers asking as high as $800 for a ready built gas motorized bike. We're talking the smallest motor on the cheapest Wallmart bike and not much else by way of aesthetics or performance.
Your seller has a few aftermarket bike parts as well as having installed a motor. I'd check on the size and how many miles he has on it. Also check on the bike brand and where it's available. Kmart or Walmart, Sears or a dedicated bike shop. Knowing the cost of the bike, motor and aftermarket parts then comparing it to what you had in mind will help you decide if it's worth it to you.