So what's to keep me from giving you $5 for your ballot? I'll fill it in and mail it in.
I'll bet over 50% of Americans are so disenfrancised with the election process, that they would sell their ballot for $5. And don't go doing the survey on the forum, because we're well educated, concerned citizens.
Besides, if you won't sell it to me, then I'll just steal it from your mailbox (or the local postal center).