It is a valid concern, but I think as usual it is more about if this data in
possibly available, not if there exists such a service as this. Because if it is, then there surely is somebody, who has this information, as thus possesses power that you (and me) do not possess. In that case I'd argue that public availability is somewhat better. In the end, you do know that somebody is buying votes, it is just that you cannot do that. It is very much possible that they already have closed back-office system like this.
It concerns me more that "politician" is a very ambiguous word. Basically everybody active enough is a politician, even though he may or may not be in senate, belong to some party and such. And an open dataset on "pretty much everybody" seems a bit more questionable than on those we consider "being in charge".
