Online auction site eBay today announced that it will extend its recent Instant Sale promotion by two weeks. Instant Sale was intended as a way of allowing users to trade in their devices for guaranteed cash.

eBay will continue the limited-time $200 cash guarantee for select functional smartphones, including any functional iPhone 3GS, Motorola Droid X, HTC Evo 4G and Samsung Galaxy S Vibrant models. A functional iPhone 4 could fetch as much as $400-$500, depending on internal memory and condition.

The company has been promoting Instant Sale since the Verizon iPhone launched, allowing consumers to lessen the impact of early termination fees incurred when switching from one carrier to another before their contract has expired.

eBay says that the iPhone 3GS 8GB (AT&T) has been the No. 1 trade in. The iPhone 3GS 16GB (AT&T) was second, followed by the iPhone 4 16GB (AT&T), iPhone 4 32GB (AT&T) and the Motorola Droid A855 rounding out the top five.

Instant Sale has generated over 1 million trade-in offers since Feb. 8, 2011.