WOW. Thanks for your super quick reply. Really appreciate it.
I had another "classifieds" plugin installed before yours but it not really did what we needed.
However I found a way with some if conditions in the template when the poster choose the option to sell only to members to show the "buy" button when somebody belongs to the "members" usergroup and if he was not it would show a button "only for members" and not clickable.
You know your templates better then everybody else and maybe you can PM me how much it would cost to implement this little piece of coding?