If your low to mid level adventurers aren't bothering to loot the non-magical bows, crossbows, and any armor heavier than hide, then in the players' opinions they're doing just fine in terms of wealth. You may consider upping the difficulty of the encounters a little bit; once they face an encounter or two where everyone's using wands of fireball, flaming burst swords, invisibility potions and exploding crossbow bolts, they'll get the idea. And if they don't loot everything I just listed, just throw it at them again. That's all stuff that a level 7 character can reasonably expect to run into and most of it is cheap, expendable "cost of adventuring" type stuff (not the flaming burst sword, of course).
Of course, sometimes it's fun to run a campaign where player inventory lists look like "Club, improvised; crude, improvised hide armor; The Guard's helmet; Leather belt with belt buckle, 5' long; 3 arrows (no bow, mind you)"
Sample encounter to help gear them up at level 7:
Chaotic Evil level 6 elf bard with mithril breastplate, flaming crossbow, 10 bolts of +1d6 frost damage, potion of Eagle's Splendor x3. And a masterwork rapier. Bard optionally has some crossbowmen following him, who would each get one of his frost bolts.
Neutral Evil level 7 human cleric with full plate armor +1, tower shield +1, poisoned warhammer+1. Cleric has 3 skeleton minions and a bunch of necromancy feats (Skeletons do 1d4+3 damage, +1d6 negative energy; explode for 3d6 negative energy radius 30' reflex DC 16 for half, and negative energy heals them; 2 hd each). Just for fun give the cleric a couple of invisibility pots too.
Chaotic Evil ogre fighter 2 barbarian 2, large +1 chainmail, large +2 flaming spiked chain (http://www.giantitp.com/comics/oots0216.html
The level 5-7 evil wizard is going to be in his shop making a doomsday device. After all, we want the players to survive right?