Is it possible through the .htaccess file or alike to make the URL string lowercase even if the user puts in capital letters in the string? For example, say someone enters "example DOT com/SELLING", is there any way that i can force the URL to become "example DOT com/selling"?
Yes, it is possible. Check stackoverflow.com/questions/3952144/quickest-way-to-rewrite-urls-to-lowercase for details.