Well, here's my story on this topic.
Found a rare staple tie on ebay listed for $200. Seller had OBO option, so I offered $100. He/she accepted (to my surprise).
Long story short, I'm sure alot of sellers inflate their 'buy-it-now' price while actually expecting a much lower 'real price'. Don't know if that's the case here - maybe they just wanted to unload the tie - but I was still surprised my offer was accepted.
I actually used to do the same thing all the time. However I've had several sellers simply ignore my offers because they thought it was an "insult" when I was just trying to get an idea of where they are at. The bad part about it is you can't make another offer until a day or the offer is countered. I had one guy end up selling a pair of jeans for $20 lower than I was willing to go because he simply ignored my initial offer that was exactly 50% of the BIN price.