Voters can sell their vote with or without a receipt. Furthermore, they can sell with a reasonable degree of certainty (if the buyer demanded it) already by submitting to a lie detector test.
There is no valid reason, moral or technical, for preventing voters from verifying their votes were accurately counted, and verification does not enable any new crimes – but it does prevent the current crime where someone's vote is either miscounted or not counted at all.
If voters are given a receipt, buyers of votes can demand to see said receipt to verify they voted the correct way. boss/abusive family member/mafia can demand to see the vote receipt on threat of your job/safety/family returning safely tonight. The potential for a vote receipt means that these 3rd parties can reward/punish you based on the way you voted. Without it, they have no way of knowing if their coercion/blackmail worked or not.
Personally, I would like to have a receipt because I think the danger of my vote not having been recorded is greater than the danger of someone demanding my vote receipt off me.
Without it, they have no way of knowing if their coercion/blackmail worked or not.
Sure they do: use a lie detector and ask them.
Honestly, the situation you describe is the problem, not the presence or absence of verifiable voting. If you've got the mafia threatening your family, voting is the least of your problems.
What is this mythical lie detector you speak of? No trustworthy lie detector exists. If you disagree: does yours work for all mental variations (psychopaths, autism, retardedness, ...) and physical variations (Down syndrome, a score of other genetic abnormalities, ...)? Have you actually verified the research or trust someone who did?
No lie detector exists that is effective to the standards we demand in a court of law, but that doesn't mean there isn't one effective enough for the purposes of an organized crime syndicate looking to buy votes.
Make the receipt optional for each voter. Assholes will still be assholes regardless of the existence of receipts. If someone is extorting you to vote a certain way, I doubt the outcome of an election will affect your life very much. You have bigger problems.
