There are a number of things with this that should be looked at. First though, lets look at your query. What is "seq" supposed to be? So no matter what row you check, it deletes all from bookmark where seq=True.
Now
echo is not a function, it's a construct, so lets start using echo like this
echo "vars"; And the naming convention on the check box, thats your main problem I believe you want something like this?
CODE
echo "<input type='checkbox' name='deleteme' value='".$seq."'/>
But I imagine what you want to do is delete each row that is checked. Assuming
$seq is equal to an ID column, then use the above checkbox input except replace
deleteme with
deleteme[]Then on post just cycle through the array deleting records that need be , or for better performance wait till you've recursed through the array and construct and sanitize, one query to delete all the rows.