Hi,

Lat time I was changing my brake pads and when I fitted new pads in, noticed that rear driver side caliper was not releasing.

I bought a second hand caliper from ebay and it didn't release either. any idea what is wrong?

Could be seized? I presume you're aware that the pistons have to be wound in using a wind-back tool? And you need to make sure your handbrake cable is adjusted right off before starting (the adjuster is behind and between the front seats).