in all the past , his owner open a shop .and to buy the pokemons items you must complete the daily quests that the gastlies give .those give gps
the items range from fossils,evo item,gems, chest and keys from past event, finally rare pokes, and exp candies

the types of quest range how much gp you get
easy 10gp
med100 gp
hard 1000 gp
challenge 3000 gp
very hard 6000gp
insane 7000gp
impossible 8000gp
insanely impossible 15000gp
nightmare 250000gp
you can suggest what the players do on these dqs

That is a good idea, but what kind of Quest/Tasks are talking about?

here is my ideas
battle certain npcs or/and elite trainers , no. times
catch different pokemon , no. times
reach a certain no. at snorelax or mew towers
buy a certain item (not in ghastly shop),or pokemon
pvp a certain no of players
play a certain no of slots ,hangman or flip coins
win an auction
lvl up your trainer lvl no of times
you can suggest more ideas