haven't thought this through, but how about a whole separate forum called "Buying a New/Used Bike" with sub-catagories called "Under $500", "$500 to $1000", "$1000 to $2000", "Over $2000".
that way before you even post you're face to face with separate threads that are too much like the one you were about to start to be ignored. i'd think that if the boards never got that many new posts it'd just be proof that it worked because it's the same stuff over and over.