If there is a better game already made, then do not bother.

I was involved in making an item shop simulation game almost four years ago.  It was basically the shop portion -- the original grand idea (which was cut down) was to have town and dungeon exploration as well.  But then recently Recettear came out, not to mention the Atelier series of games were translated, the most recent one being Atelier Rorona.  They both epitomized the ultimate 'item shop RPG' experience.  I actually prefer Atelier Rorona not just for its art style but because the spirit of the game was more in tune with our original game concept... namely fulfilling orders by crafting/synthesizing items, rather than buying/selling to maximize profit and meet debt which was Recettear's main theme.

So now that I believe that the ultimate item shop game was already made, I no longer feel compelled to pursue any item shop game in the foreseeable future, since the market was already satiated.

I've been playing some games recently as a form of 'market research', and to double-check my own design ideas against what has been implemented already.  And sure enough, if your idea is good, then it has been done already.  For example, Grotesque Tactics was the best example of an isometric RPG game that transitioned well from free exploration mode to tile-constrained movement during the turn-based combat phase.

So I already encountered a game that already has the mechanics I envisioned perfected.  But I'm still going to go ahead with my game, because it has a story that just begs me to tell it.  I'm still going to mix and tweak some mechanics though just so that the experience flows a bit differently.

At this point, I'm not going to meet the original deadline I set for myself, but I am still motivated to finish this game.  It's essentially my testimony to the world.