It is common for roadblocks to be erected on election days in order to stop/search cars with bundles of cash in the back to pay the non-enfranchised to vote for a particular candidate. Sadly, lack of political awareness means many are ok to give their vote for cash.

The point of a secret ballot is that there is nothing to stop them taking the cash, then going to the polling station and voting for someone else. Are the ballots not secret in India?

They are. People sometimes take money from multiple parties at the same time. However many times largest spenders get the max votes. It is just not money, gifts including alcohol jewellery etc are given, so it mix of relationships with grassroots party representative(inc caste equations) and the gifts. If the sentiment is really against candidate no amount of money help.