Problem not solved. A savvy attendant just checks the tickets on the end of each roll. If it's a winner, go ahead and buy it. Otherwise, wait until a customer buys the loser first.
Disallow attendants to buy tickets while they're on the job. [However, they could signal accomplices to buy it for them. It would take a team, then.]
Most lottery tickets are sold from small convenience stores, either family owned or with just a couple of staff - hard to enforce this.
There is a crackdown on relatives of store owners winning the lottery - but is normally them defrauding winners who come in to check the ticket.
I grew up in Maine and tickets are dispensed this way. Not sure about returning tickets, but knowing Mainers, I'd be surprised if they would take them back. It doesn't pass the straight-face test.