How does selling votes work? And how much do you sell them for? How do you know how many votes you have/can sell? 

Basically, another player's tells you what club they are running for presidency in, you go to that club page, click the "vote" button next to their name, and they send you whatever the agreed on price for your vote(s).

I usually sell mine for 500k per vote or 1mil per exclusive vote. The difference is just that if the other person asks for exclusive votes, then you can't vote for another player running in that same club for that month.

You can vote in every club each month.

Ohhh ok. Is voting all month or towards the end of the month?

You can vote at any time. You just see people advertising for it more at the end of the month (trying to get as many of those last minute votes as possible).

I would also like to add that you must be upgraded to vote, so however many upgraded accounts you have, that's how many votes you have :)