First, remove ALL references to the shop in the MySQL database. That includes the shop_items and shop_inventory tables (it seems that you've already done that). You'll also need to remove:
* the money and moneyBank fields in smf_members
* every shop entries in the shop_settings table (shopVersion, shopBuild, shopCurrencyPrefix, etc...)
* the countCredits (or similar, can't remember at the moment) field from the boards table
Next, unarchive the shop tar.gz file. Put the installScript.php file in your forum's directory, and run it.