I just ran into the same problem. I had to modify my htaccess file. This thread showed me how.
So my finished htaccess file that now works for wordpress as well as other pages on my site looks like this.
RewriteEngine On
RewriteBase /
#
RewriteRule ^buy-(.*)-(.*)\.htm$ /morenow.php?title=$1&itemid=$2 [L]
#
# BEGIN WordPress
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
# END WordPress