eBay is where you'll likely get the most for it. I've seen them still going for a little over $300, so use a "buy it now" for like $325.00 and let people make offers on it. That's, if it's in great shape, you have the original box and all that.
I would ship to US only, accept paypal from "verified accounts" only, and make sure to ship with delivery confirmation and insurance.
Edit: Do NOT sell it for $150-200 unless it's in bad shape. You're selling it as a non-contract obligated item, so that's why you get more for it, than AT&T sells it for, new with contract. I just sold a white 3G for $225.