I bought 10 stacks this morning before leaving for work. I won't be back home until around 5pm and probably won't have a chance to check my character and buy more tickets until at least 6pm. I'm hoping those 10 stacks will last until then.
Basically, don't worry about over-buying. If you have the cash to buy 20-30 stacks, then buy 20-30 stacks and forget about them. You can always sell the unused stacks back to the merchant for the same price you paid for them.
(BTW, I had 12 stacks of tickets when I checked the computer before driving off. I think i picked a good ring.

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