Items in the Retroverse are powered by
the Gifted One using them. As such, their
power is limited by the power of the user,
as defined by their adventurer's level.
You can use more powerful equipment, however
the effective bonuses/values you get are 
limited as follows:

Weapons
---------------
WC: level * 3
Special attack: level * weapon_skill_percentage * 2
Stat bonuses: level/3
Skill bonuses: level/3
Shield PV: level * 3
Non-shield PV: level

Armor
---------------
PV: level * 3 percent of max allowed for slots/bulk
Stat bonuses: level/3
Skill bonuses: level/3
Resists: level/3

Note: Negative stats are not limited by level