If both parties know how steemit algorithms work then there is no real point in offering money for a vote.

1. Voting ( curating ) will earn money for Voter as subjective contribution to the system.
2. So the question is what the monetary value is, and why the voter decide to sell it outside the platform.
3. Larger the SP more weight the voting carries and these users have better knowledge of how the voting works , unless you pay above the odds they won't sell their votes.
4. Some users without knowledge could sell their votes for a short time before they figure this out , but that makes no difference due to lack of SP in those accounts.