Or, rather, what it should not be. Vote buying is a perversion of democracy. It is a system in which groups of citizens try to force the government to take decisions that correspond to their self-interest and that give them certain advantages, such as tax breaks, subsidies etc. Citizens try to force politicians by giving or threatening to take away their votes. They desire something and the price they pay is their vote. They give their votes and expect to be compensated for this. They sell their votes and their electoral fidelity for certain advantages. Citizens have votes and politicians need these votes; politicians have access to government-provided benefits and citizens need these benefits. Hence, it is natural that exchanges take place.
However, this kind of logic is of course detrimental to democracy. A first problem is that a benefit for one group is always at the expense of the rest of the population. The financial loss that results from a tax cut or a subsidy for a certain group, has to be compensated by increased contributions by the rest of the taxpayers. The advantages given to some people in return for their votes are, of course, not paid by the politicians themselves, but by the rest of the population. In fact, politicians buy their votes with the money of others. It is often the less vocal and hence those already disadvantaged who end up paying the bill.
Because politicians run the risk of losing power when voters go elsewhere to sell their votes, we often find politicians outbidding each other and promising ever more important benefits, to preserve or conquer power. They have to buy votes and fidelity by way of more and more benefits. Politics becomes a kind of inverted sales. Politicians have to grant ever more important benefits to entice voters in their camp. Votes are scarce and demand is high, especially before election periods. If demand is high and if there are several competing buyers, then prices go up. Votes become ever more expensive because buyers can be played off against each other.
This results in budget deficits, an over-sized state, and dependency. An economic logic is applied to politics, which as a result loses its identity. Democracy degenerates into an economic system in which groups of citizens use the competition between political parties (the competition for the votes of the citizens instead of the competition for the money of the citizens, as in the real economy), in order to achieve as many material benefits as possible in return for their votes (their only political capital). The political process has become a market process. The laws of economics (offer and demand, free competition etc.) take over politics.
Of course, it is true that democracy is a free competition between would-be leaders searching for as many votes as possible, but it should be much more than that. It should be a place of debate and discussion, of freedom and equality, and of the pursuit of the general interest.
In a system of vote-buying, politicians represent groups and interests instead of society as a whole and the will of the people as a whole. Resources are redistributed, not from the rich to the poor, but from everybody to those who are vocal and have the best bargaining power. Transfers depend more on electoral importance than on real needs.
Direct democracy is the only solution to the problem of vote buying. In a direct democracy, there are no representatives who have to grant all kinds of benefits to pressure groups, in order to cling to power.