I'd imagine scratchers would be almost impossible unless you could somehow get all the tickets from every place they are sold. I'm not into gambling, but I guess it might work if there is X number of prizes/money per roll of tickets.
Even if you can't buy every ticket, there is well-established math about how to optimize profit from a venture with known risk and reward, and the math does not require you to exhaust the statistical universe.
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In most cases, there is, which is part of why a huge percentage of scratchoff prizes are won by workers at the place that sells them. Most players will scratch and redeem their prizes right in front of you, so if you watch a certain number of scratches occur in a roll and you know the prize structure of the particular card, you can calculate how many non-winning scratches you need to see for the odds to be in your favor.
I looked into this a few years ago and considered starting one of those stands that sells scratchoffs to do just this, but decided a) it wasn't quite lucrative enough to be worth it, and b) I wasn't sure of the ethics of skewing the odds against your customers like this anyway.
This is interesting because I don’t think anyone would view the store as unethical for continuing to sell tickets from a roll when they know there have already been X winners from that role and therefore customer odds have gone down.
The problem with selling out the roll when winning tickets have already been sold only occurs in tandem with the retailer buying remaining tickets when only non-winners have been sold so far. These aren't separable situations.
They absolutely aren’t trying to buy them all, that would just be a guaranteed loss, since they only return about 40 cents on the dollar.