Maybe players with more DK should have a selection of more expensive weapons & armors with higher atk & def. Since they can save up more gold then players with lower DK.
However, the stock will be shorter as well. Meaning, at Daimyo rank, the number of stock for the strongest weapon might be 1. Once someone bought that, others will have to wait until the next gameday for another one to be in stock. It maybe a color-coded weapon since it's so rare.
Example:
(Academy Student)
weapon name/atk/cost
Training Kunai 1 48
Solid Wood Kunai 2 225
Rusty Steel Kunai 3 585
Dull Steel Kunai 4 990
Stainless Steel Kunai 5 1575
Very Sharp Steel Kunai 6 2250
Extra Light Steel Kunai 7 2790
Feather-weight Steel Kunai 8 3420
Combat Kunai 9 4230
Multipurpose Kunai 10 5040
Well-balanced Kunai 11 5850
War Kunai 12 6840
Golden Kunai 13 8010
Best-selling Kunai 14 9000
Star Metal Kunai 15 10350
(Genin)
weapon name/atk/cost
Training Wakizashi 2 225
Rusty Wakizashi 3 585
Dull Wakizashi 4 990
Stainless Steel Wakizashi 5 1575
Very Sharp Wakizashi 6 2250
Feather-weight Wakizashi 7 2790
Well-balanced Wakizashi 8 3420
War Wakizashi 9 4230
Well-decorated Wakizashi 10 5040
Ouwakizashi 11 5850
Expensive Wakizashi 12 6840
Honor Blade 13 8010
Best-selling Wakizashi 14 9000
Star Metal Wakizashi 15 10350
Legendary Wakizashi 16 12500
p/s: Players with kekkai genkai that doesn't need weapons on the other hand (like Jyuuken) should be able to level their skill higher.