Before the loot bug came to be.. knuckles, crystals dropped at a resonable rate, especially on goblins. Thunderstones, alch fire and acid flasks were rarer, but not so much so as to be up there with book drop rarity.
Once the loot bug is fixed, maybe the drop rate for ingredients can be upped, then all you trap-happy people can start asking around and buying them like ya should

Another side of this matter to consider is the people who deliberately use pvp to harass other players. If a trap store is available, whats to stop them from buying all the ingred's they need to lay 100 traps in and around an enemy city, or a popular hunting area? Knowing full well (at least in the hunting area) that their traps could and prob would play a big roll in getting players killed by monsters and causing them gold and xp.
Thats prob why trap ingred drops arnt as common as people would like. To prevent pvp-abuse