I really can't fathom people getting $350+ for a 16GB 3GS. $400? That's just ridiculous and very rare, unless you happen to live in a small city where there aren't many people or something.
I live in Los Angeles, and craigslist is flooded with iPhones. I sold my white 16GB 3GS for $300 - and honestly, I had one buyer contact me, and I followed up quickly enough to finally get it sold at that price. When I had it listed at $350 and then $330, I didn't get a single inquiry.
Luckily, I didn't care much about profiting or even covering the entire cost of my iPhone 4. All I'm paying is tax on a 32GB, and I feel satisfied with that. Haggling for $20 more dollars isn't worth it to me.
And ebay is evil.