Pistols are mostly bought "bad". Rifles are mostly bought "bad". Cannons are mostly bought "bad". Melee Weapons are mostly bought "bad". (There are a few normal exceptions I think).
APU Spells are bought "better". PPU Spells are bought "better".
As far as I can see there is no reason for the imbalance. Either the Spells should be downgraded, or the other weapons upgraded. Or both, and they meet in the middle.
From experience I can say that it is quite good doable to level with bought APU spells, but not with Rifles and Cannons. Since a good level experience with bought items leaves out the traders, I'd say downgrade the spells or meet in the middle.