Things to check on 2nd hand bike

Couple of general things to add if this thread pops up more regularly in search:

1. Check the tyres for cracking. If the bike has been stood for a while, possibly with a slow puncture, you might get cracking on the sidewalls. I've had this with a couple of sets of used wheels with Vittoria tyres but it's not specific to that brand. 

2. If it's a rim brake bike check the brake track for wear. There are usually wear indicators (a dimple) on the track or you can just hold a straight edge against it (bank card will do) and eyeball it.

If you're buying in person though the main thing is to ask questions of the seller. This also works online to a certain extent. How much do they know about the bike, what sort of riding do they do, etc. In my experience cycling enthusiasts take care of bikes and love to talk about it. It's not a guarantee of a good bike but it goes a long way. 

Reason for the sale? Vendor should be able to say what he's used it for, the 2000 km. If it's "dunno, I'm selling it for a mate/relative" be very wary. The main thing apart from has it been stolen is has it been crashed - as others are saying.  

Are the wheels reasonably true, do the gears and brakes work? Does the seatpost move ok? 

Apply the front brake and push back and forth for headset issues (might only be grease or adjustment needed), and wibble both rims for play. You ideally want the chain off to check the cranks - they too should turn smoothly without play. 

Expect to replace chain, bar tape and probably tyres. 

Most components can easily be replaced and offer a chance to drive down the price. I'd be concerned about stuff that would need specialist tools or a visit to the bike shop.

Is the frame good? Straight and no cracks.

Wheels? Although easily fixable, can be a ball ache.

How easy to get to the crank set and bottom bracket? Special tools needed?

Then usual sign for wear, tear and abuse as mentioned above.

I wouldn't be too bothered about wear on the drivetrain as they're mainly consumables, though you could probably drive down the price if you can show that they need replacing straight away. It's worth checking that the gears work smoothly without any sign of chain rubbing too.

As well as checking for "play" in the cranks, see if the wheels have any lateral movement as that would indicate worn bearings. Similarly, the front fork/handlebars shouldn't have any play in them (worn headset bearings) and be able to turn smoothly and easily.

Most important is to check the frame and handlebars for signs of falling damage. If there's significant scuffing on one side of the handlebar/shifters then it could well have more damage elsewhere. Check the frame for any cracks and possibly tap around the frame to help identify structural damage which would make the purchase unwise. It's tricky as you can get superficial damage to a frame/paintwork which is not a problem.

Someone will be able to give a much more detailed list, but as a starter and based purely on wear and tear I look out for on my bike:

Give the cranks a sideways shake, checking for "wobble" - a good indicator of the state of the bottom bracket.
A badly worn chain could also wear out the rear casette/chainring, so have a look at the teeth - I believe the term is "shark tooth" - the teeth will look sharp.
Wear on the disc brakes. Are they obviously thinner where the brake pads press.
Wobble in the wheels - I normal hold my thumb on the frame, as close as I can get to the rim without touching it and spin the wheel. It should be evenly spaced all the way round.