RewriteEngine On

RewriteCond %{HTTPS} !=on
RewriteRule (buyerdetails|add_buyer).php(.*) https://xxx.net/$1.php$2 [R,L]

#bad delimiter flag
#RewriteCond %{REQUEST_URI}  ! ^/(buyerdetails|add_buyer).php*

I just want to evaluate this condition IFF the previous fails...

RewriteCond %{HTTPS} =on
RewriteRule (.*) http://xxx.net/$1 [R]