The most likely answer is that you're using a third party cheat device. If you input codes incorrectly, there's a chance they could harm your gameplay. Another possibility is that you got a bootleg game which is very uncommon, but not unheard of.
In any case, try saving the game outside Join Avenue on one day the new going back in the next day (sometimes saving in it prevents the shops from resetting prices). If that doesn't work, and you have an Action Replay, I'd recommend using a code to manually reset the timer for the shops in the Avenue.
If that doesn't work, I'd recommend consulting with the experts over at GameFAQs. They have some experience with situations like this. If you can't fix whatever's wrong with you're game, you may have to restart it