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    Good question!  Maybe one that hasn't been asked yet.  Here's a start:
    
    - Frame:
        -- rust (Fe) or corrosion (Al) or fraying (C)
    	-- finish scratched
    	-- out of alignment (need not be from a crash)
    	-- crashed (bent or deformed tubes or lugs) - the biggest problem
    
    - Expendables:
    	-- tires, tubes deteriorate
    	-- chains stretch, wear
    	-- freewheels and chainwheel cogs wear down
    	-- freewheel bodies get gunked up, could corrode
    	-- handlebar tape gets grundgy
    	-- brake-hoods - ditto
    	-- cables - stretched, worn, frayed
    	-- brake-pads - worn down
    
    - Wheels:
    	-- rims out of true or (worse) round
    	-- spokes slack, fatigued
    	-- hubs: see below
    
    - Bearings & Races (bottom bracket, headset, hubs, pedals)
    	-- races could be pitted; 
    	-- bearings could need replacing
    	-- lubrication could need renewing
    
    - Other Components
    	-- Saddle: worn - cover torn, padding deflated or migrated
    	-- Brakes: out of alignment (calipers) - this can be fixed
    	-- There should be limited problem with metal fatigue in
    	   cranks, seatpost, handlebar, and stem.
    
    - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 
    
    Off hand, I'd say the most serious things to watch out for (if you
    can find out about them at all) are
    
    	- frame condition (has it been crashed? is it rusty?)
    	- bearings and races
    
    -john
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|  |     RE .0
    
    I have boughten two Mountain Bikes for my wife and I in the last two
    months. I feel I have gotten a good deal! I priced new and found that to
    get into a decent bike, you're gonna spend at least $350+. I bought a
    Specialized "Rockhopper" for myself and a Mongoose "Hilltopper" for my
    wife. Both bikes came with a few extras, locks, water bottle and cages,
    pump, toe_clips, ect. I could not afford to spend a $1000 to get into
    the sport (ah... what the hell...one more hobby!). I got both bikes for
    under $500.
      Both bikes were in great condition, a few scratches here and there,
    nothing that I wouldn't have done myself. I looked for warped rims,
    broken welds, and no loose cranks or axels. Just look the bikes over
    well and test ride! There are decent used bikes out there, just be
    patient and wait for YOURS!
    
                                           Jeff
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|  |     
    I don't know if there is a standard formula for depreciation of
    bikes, but it seems that the low and mid range, at least, drop
    in value rather quickly.  Is there a "blue book" for bikes?
    
    I've bought my good bikes at end of year close outs, and saved
    hundreds.  But if I were in the market now, I'd probably try to
    buy something in decent shape from a private party, and save
    even more.
    
    	Dave
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