But what Earthly good is it to use that phase at all? I mean really. You want $3750 for your piece of crap Plymouth Valiant? You put "or best offer" and you've just guaranteed that you won't get full price. You've already told them so.
Just who's best offer is it? It's certainly not yours. Yours would be some astronomical number above asking price. Theirs would be for "free". The actual price is somewhere in between and you've just given them an immediate coupon.
I guess you could jack the price up unrealistically in the first place and then expect the "best offer" to come in at what you wanted in the first place. That technique sometimes works in Pawn stores I suppose.
So next time you sell something...don't look stupid and add O.B.O to your item. Do like everyone else: Overprice it and sell it on eBay!