How much is your old iPhone worth?

On eBay today there are thousands of unlocked first-generation iPhones being offered for sale -- individually or in lots of 10 or 20 or more -- for prices ranging from about $450 to $700 per 16 GB phone.

But if you want to sell yours, you'd better move fast, say people familiar with the business of buying and selling used iPhones.

"I think the eBay market is going to dry up pretty quickly," says Aaron Vronko, service manager for Rapid Repair, an iPod and iPhone service bureau in Kalamazoo, Mich. "The market is very different today than it is going to be a month from now."

What's changing is Apple's (AAPL) new iPhone 3G, which goes on sale on July 11 for $199 (8GB) and $299 (16GB) in the U.S. and is being offered for free or nearly so in Europe (see here).

As Engadget's iPhone 3G FAQ puts it: "The funny thing about a $200 3G iPhone is that it makes it pretty difficult to sell your original iPhone."

Engadget suggests that if you don't know someone who doesn't need 3G, you might want to keep your old iPhone as a backup or unlock it for use overseas with another carrier's SIM card.

Gizmodo's FAQ points out that your old iPhone will still work as a cheap iPod touch, even after the SIM card has been removed. "Everything but the calling features should be active, except it will say 'No SIM' at the top."

Rapid Repair's Vronko thinks there will be a significant after market in these re-purposed iPhones. "The original iPhone is a really capable off-network device. It's got a more powerful processor than the iPod touch, a speaker, a microphone, Wi-Fi and Bluetooth headset support. If you unlock that iPhone and get a VOIP (Voice Over Internet Protocol) client, you might even be able to use it as a phone.

So what can you get for your old iPhone today?

If you don't want to take your chances on eBay or Craigslist, there are any number of resellers that will take it off your hands. BuyMyTronics.com, for example, will give you an immediate quote based your answers to their online form. For a 16GB iPhone in good condition with the box intact and all the extras (including working earbuds), they'll pay $190.92.

That's if you sell it today. After July 11, all bets are off.

Rapid Repair, which has been buying broken iPhones and refurbishing them for resale abroad, hasn't set its post July 11 price, but it's looking at somewhere between $50 and $75 for a working phone.

With 6 million first-generation iPhones out there, many of them purchased by early adopters who will want to upgrade to 3G, Vronko is pretty confident he'll get thousands of takers.