Is it worth to buy used laptop for an emergency use?

It should be okay as long as you can check the laptop or the proper functionality. I would, in fact, advice you to get the laptop in question thoroughly checked by an expert. It would be better if the concerned expert is someone you are acquainted with.
In addition, if you are getting a good price you can also neglect minor issues (that can be solved) so that you can get it repaired at your own cost.
I would in your place be better off by getting the party concerned sign an agreement of sale with a few conditions mutually agreed( I doubt it is possible in India).
Finally, I would advice you to buy from a reputed store rather than on OLX or Quickr as you can be assured of the repairs if any at a reasonable charge.

Used laptops have their shelf life extended and then resold. So it is not wise to buy reused laptop for the emergency use. Because such laptops are likely to end up the same way like any other machine. And the unpredictable short life of the used laptops makes you not to trust them for emergency use.

Instead of buying reused laptop for emergency use, you should buy netbooks for the emergency. The price of netbooks is under 18000Rs. So at the half of cost of laptop you get those netbooks. If your book does not involve Windows specific task then you can also buy low cost Chromebook. Chromebooks are perfect for emergency use if your work involves only email and other internet based tasks.

I read the above responses from experienced member and would like to follow the same. I also felt that it is better to buy a low cost new laptop as an alternate one instead of old or used laptop which may or may not work for a long or as expected. The new one at least will have 1 year manufacture warranty and replacement duration etc.