This thread shows OBO doesn't mean the same thing for everybody. To me, it means the seller is willing to negotiate.
I know I risk insulting a seller if I make a very low offer. Worse than no response, it could potentially cut off any future negotiation, even if I was willing to pay the full asking price.
As others have said, best to show interest without discussing money to get an idea of where the seller stands and show him you're serious about the game.
If somebody gives me a low ball offer with no discussion, the first thing I think is he's a flipper.