Earlier this week, eBay (EBAY) announced a smartphone trade-in promotion with its Instant Sale program pegged to iPhone 4S. Smartphone owners with qualifying smartphones can snag hundreds of dollars in cash: an AT&T iPhone 4 in excellent condition snagged up to $357, while an AT&T iPhone 3GS got as much as $229. Android smartphones like the HTC Sensation 4G and HTC Thunderbolt were valued at up to $225 and $200, respectively. The first phase of the promotion ended last night, but the company is accepting trade-ins through Oct. 18, with trade-in values varying day-to-day.

The company reports more than 50,000 trade-in requests from smartphone owners, or one offer every two seconds, with 90% of those being iPhones, and the rest being Android and BlackBerry devices: 65% were iPhone 4s, 30% were iPhone 3GS units and 5% were iPhone 3Gs. According to a spokesman, eBay expects offers to keep coming in and match, if not surpass, a similar Verizon iPhone promotion it ran last February when consumers requested to trade in 130,000 smartphones.

Ian Bednowitz, manager for eBay Instant Sale, says the promotion is one of the most competitive out there. (At the very least, it gives more cash back than Apple’s own Reuse and Recycling program, which offers up to $200 for an iPhone 4.)

While the company’s trade-in data obviously isn’t representative of overall demand for the iPhone 4S, and eBay wouldn’t directly comment on whether the data bodes well for the phone’s future, it could be viewed as a good indicator that some consumers — or at least some Apple early adopters — want the 4S and want it on Day One, even if it means the phone they’re getting physically resembles the one they already have.

That sentiment would differ somewhat from the perception in some corners of the media that yesterday’s unveiling was somewhat of a disappointment, and that the iPhone 4S might not sell as well as iPhones past. Gartner analyst C.K. Lu told Reuters that he believes users may wait until the next major iPhone hardware update or else shift to other brands with more advanced hardware specifications. Meanwhile, former Business Insider editor-turned-one-man blogger Dan Frommer took the opposite tack: “If you’re disappointed by the iPhone 4S, you’re nuts.” And optimistic forecasts from the Taiwan-based component-tracking group TrendForce predict the 4S could boost overall iPhone sales by 22% over the next 12 months.

Regardless of whether the iPhone 4S sinks, swims or soars once it hits the market, a good chunk of upgraders will walk away from eBay and Apple feeling a lot less strapped for cash.