First: I don't get this:
$street_number=ereg_replace("'","''",$_POST['street_number1']);
do you mean addslashes($_POST['street_number1']) ??
You can't successfully put named array elements in quotes in PHP
echo "$array[0]"; //this works
echo "$array['named']";// this doesn't
so either assign the (1) values to non-array variables OR (2)
concat the strings.
Also you can't just add quotes within quotes: you need to escape them
echo "\"Hi!,\", he said."
so this is basically mostly wrong:
$sql = "INSERT into listings (street_number,street_name,postcode,...
values('$street_number', "$POST['street_name']", "$POST['postcode']", ...
$street_name=$POST['street_name'];
postcode=$POST[postcode']";
$sql = "INSERT into listings (street_number,street_name,postcode,...
values('$street_number', '$street_name', $postcode...
OR
$sql = "INSERT into listings (street_number,street_name,postcode,...
values('$street_number', '".$POST['street_name']."', '".$POST['postcode']."', ...