Is it a bad idea to buy a used bike that is somewhat expensive ($1000 range) if you don't know much about bike mechanics? Of course you can find a checklist online of what wear/damage to look for, but is it better to buy a new bike if you are not an expert in looking for this kind of stuff?
I don’t think it’s a bad idea, but be prepared to repair stuff.
Things even a novice should be able to check:
- Frame and fork without damage (i.e. no cracks or dents, nothing bent). Small paint chips or scratches are okay.
- Shifting to all gears works.
- Braking works.
- Wheels are true, no damage to the rim or spokes.
This ensures that the most expensive components are in fairly good shape. You might have to replace a few wear parts sooner or later.
Things you are likely to miss:
- Drivetrain (most likely the chain) is worn. A new chain costs ~20€. Cassette and chainrings are more expensive but last >10Mm.
- Cables+housing are worn and have high friction. New cables cost ~20€.
- Bearings are worn or have play. Greasing&adjusting is free but new bearings (or new components) can be a bit expensive.
- Brake pads are worn (or bad). New ones cost ~20€ (per set).
- Tyres are worn (or bad/inappropriate for your use). Check for a tread wear indicator (TWI) mark. New ones cost 15 – 40€ (per piece).
- The braking surface of the rim is worn down. Only applies to bikes with rim brakes. This is the most expensive wear part since it requires you to replace the rim and rebuild the wheel. Sometimes it’s easier and cheaper to replace the whole wheel. Some rims have a wear indicator otherwise it’s hard to check how much braking surface is actually remaining.
- Suspension has excessive play (if the bike has suspension). I don’t know enough about mountain bikes to comment on this.
- Per Adam’s comment: Old or exotic components which can be hard (or expensive) to replace if/when they break. Unconventional wheel diameters on vintage bikes come to mind.
Edit: Oh, one thing I forgot, which is actually the greatest risk:
- The bike doesn’t fit you (wrong frame size) or is totally unsuitable for what you want to do. For example if you bought a full suspension mountain bike or a city bike but actually want to ride quickly and efficiently on smooth roads. In that case you can really only sell it on and hope you don’t lose money.
Rule of thumb: if you are an expert, you may be able to buy for 50 what is worth 200/500/1000, your experience will make up for the 150/450/950 gap.
If you are not an expert, you will for sure buy for 1000 what is worth 100/200/500. You will learn experience points worth 100/200/500. And you will lose a lot of money.
Yes, you may be lucky buying a good second-hand bike for cheap, but it boils down to three possible paths:
- being lucky (what's the last time you won the lottery?)
- being a bit naive and buying stolen stuff without understanding it (and now that I told you, you cannot anymore be naive saying "I didn't know about that" ... hey, your fault, asking questions on the internet you will grow up a thick skin :D );
- knowing your market, either having experience in the mechanics of bicycle or knowing where rich/middle-rich people of your region sell their used stuff when they need space in the garage.
Another potential risk with buying a used bike is that it can be tricky to tell if you fit the bike (or if it can be made to fit you by changing the stem length/position and possibly the handlebar). If someone is a novice, I'd recommend not doing this unless they can get an experienced friend.
The possible pitfall of relying on an experienced friend is that experience with cycling is no guarantee that they are good at elementary bike fitting. I could probably help someone fit a performance or endurance road position, but I'd be a lot worse at a more relaxed drop bar bike position or anything to do with flat bar bikes.