Buying a second hand bike might not be a bad option if you are careful in selecting a good bike that is well maintained and if yiu buy from a certified seller.
There are certain things that you need to keep in mind before you set your eyes on a second hand bike.They are:
1) Overall condition of the bike, its model( well do not prefer a very old model unless you want that specifuc model) and its functionality.
2) The working condition, the number of kilometres it has run so far and the tyres.
3) Make sure all the documents of the bike are available and perfect.
4) Make sure it is running well and always buy from a authorised seller rather than a individual ( this depends) .
All I would like the suggest is, if you are planning to have a bike for a short period, say a year or so, then you can go for a second hand vehicle and then sell it off.But if you are havibg any long term plans of having a bike then I would suggest you to go for a new one.