Flenix

Ok so I've had a GTL question bugging me for a while; Why do people so often sell things higher price than you could just buy it from a store for? For example, pokeballs. They're $200 in any town's pokemart... so why are they always $300-$600 in GTL? Surely I'd just think "Well, I'd have to collect it from the PC anyway, may as well buy it from the NPC" and save myself hundreds.