If you want the latest and greatest, yes, sell it as soon as you can.
The closer we get to the rumored September 12 launch date and the September 21 sale date, the less people will be willing to pay for your soon to be outdated iPhone. As of now, assuming your iPhone 4S is in good condition, you can expect to sell it for around $400 (16GB), $450 (32GB), and $650 (64GB) on eBay. Looking at historical trends, your iPhone 4S’s trading value will diminish by about 3% each week leading up to the availability of the new model. Once the new model is available, your phone’s value will diminish at a lesser rate. For a look at the selling prices of different iPhone models last year, check out the graph in this article from CNET.
If you look at the graph in the aforementioned link you’ll notice that the iPhone 4S lost $70 in trade in value on September 21, 2011. That’s the date when All Things Digital, a blog affiliated with the Wall Street Journal, confirmed that the new iPhone would be unveiled on October 4. Because that was the first confirmed date from a very reputable source, prices fell. All Things Digital will likely confirm the release of the pending iPhone 5 any day now. And, once they publish their dates, your iPhone 4S’s value will lose about $70 in trade in value.
The problem with selling your phone on eBay is that you’re going to have to use another phone before the iPhone 5 actually comes out. Gazelle, one of many gadget trade in sites, lets you claim an offer and gives you 30 days to ship out you iPhone. For example, if you have an iPhone 4S (16GB), you can claim Gazelle’s $277 offer and wait 30 days before you ship it out to them. With Gazelle, you are selling your phone for less than you would on eBay, but without the hassle of switching to another phone for a month. If you have a cheap phone to use for a month, you’re better off selling your iPhone 4S on eBay.