Those ebay purchase units under USD$20 are generally clones using OEM or third party PL2303 chipset.
Prolific from Taiwan, who made the original dongle, prevent these clones from using its drivers and placed some checks in the driver installation, and these clones will not work.
Luckily the clones makers do provide some forms of drivers, generally work for WinXP. If you got the older batches, probably would not have the drivers for Win7.
Thats why some folks had issue with these clones.
A piece of advice, try to download all these similar drivers and install via trial and errors.
Do take note that u would need to reboot on each driver installation, and removal before installation of next driver.
My 2 cents worth..