I expect that you mean your country is Nigeria, since it is the only country, as far as I know, in which presidential elections are held, which began yesterday, Saturday. Isn't it?

In the Nigerian example, the hypothesis of the candidates' offer to buy votes is very candidate, since there are 18 candidates competing for one seat (presidency). Of course, this falls within the list of political corruption, because a person who works to buy votes from voters to ensure victory means that he is not eligible for that position.
As for the voters, they are also involved in corruption if they accept to sell their votes at any cost.