Saiga12noob

Hello, New to the site, I looked around on here but didn't see anything about this but if i missed a post about it I'm sorry. I've been looking at 12s, watching the auction sites. I see guns that are exactly the same. (19" NIB, nothing done to them) but some are bid up to $800 while others start out at like $675 but no one bids on them. So my question is WHY? What am i missing?? Thanks