With regard to quantity buying, why not implement a discount system? Off the top of my head, here's how it could go:
- Seller puts up 99 superstuffs at 100 gil apeice (total for stack = 9,900)
- Game asks "Would you like to offer a bulk discount, yes/no?"
- If Seller answers Yes, the stack must be sold as is (any Buyer must buy all 99)
- If Seller answers No, the "standard discounts" will be displayed on the Market Board
The "standard discounts" would be
- Buyer buys 1-10 items, pays full price (100 gil per item)
- Buyer buys 11-25 items, receives 5% discount (95 gil per item)
- Buyer buys 26-50 items, receives 10% discount (90 gil per item)
- Buyer buys 51+ items, receives 20% discount (80 gil per item)