To uninstall SMFShop, the main thing you'd need to do would be to delete all the SMFShop related stuff from the database (smf_shop_* tables, shop variables in smf_settings, money and moneyBank fields in smf_members, and countMoney field in smf_boards). You do this through a tool like phpMyAdmin. It doesn't matter about the files, as they'd be overwritten when you reinstall.
Once you remove all the SMFShop stuff from the database, try installing it again.