LittleMe, I don't see that as much different from when a merchant offers an individual item or storewide sale. Sucks to be me that the item I previously bought can now be had for much less, but that's the nature of retail.
If you want to make an item available as a one-off, that's your prerogative. My point was let the customer be the person who makes the decision on what they wish to purchase of yours - you know, regular vendors. Instead of continually feeding a gacha in the hopes of getting that one item they desire.
Sellers, an even better idea for you. Have everything you make available for purchase at what you deem a fair price. And let me, the customer, decide what i wish to purchase. Retailing 101, it's not hard.