[Porsche-O-Phile]

Radiant barriers are legitimate methods of increasing envelope insulating value. I would be EXTREMELY skeptical of anything on ebay - it's a haven for scam artists and snake oil salesmen. As dad911 says, you'll also likely run into issues with documenting the energy code compliance based on a "well, so-and-so SAID so". They very often want manufacturer's cut sheets, etc. - something with a little bit more legitimacy.

Edit: different municipalities will respond differently to these and give different "credits" for using them. It's not necessarily a fixed number - talk with a building department official.