#) It seems like you're not sanitizing the input parameters before using them to construct a SQL query. You should propably fix this.

#) You shouldn't output the SQL error messages to the user.

#) The purchase column shouldn't have a sort icon if it isn't sortable.

#) IMHO the back button doesn't add any value since it just duplicates browser functionality.

Another little nitpick‎: The sort icon on the page is a image ;-)