What's to stop me from filling it out the way I said I would, going to the voting office and saying the ballot I had previously was lost/damaged/spoiled and then voting differently than how I was paid to vote?
The premise behind a secret ballot is it can't be proven how you vote. You can still sell your vote (illegally), you just can't prove you voted the way you said you would. As shown above, absentee ballots don't remove that distinction. I suppose the criminals could just trust each other...
This topic has been discussed for a couple thousand years now, and you clearly have access to the internet. Feel free to search for the answers on your own.