look closely at the link on the gear.php page:
<a href="detail.php?ID=<?php echo $row_rsItems['ItemID']; ?>"><?php echo $row_rsItems['ItemName']; ?></a>
it is linking to the detail.php after.php, you can see a question mark followed by "ID" this is the name of the querystring variable being passed.
the value being passed is the ItemID from the recordset:
ID=<?php echo $row_rsItems['ItemID']; ?>
this creates the link as:
On the detail page, double click the recordset and inspect how it is created to filter the ItemID column on the ID querystring variable.
the issue on your page, is that you are not passing a name for the querystring variable
only a value.
If you are using the same details page from the eCaart sample files, you just need to modify your links to pass the value using the name ID: