Apologies for the delay, your thread was missed in error.
1. Yes, you could do some minor customization and remove the contact form in the sidebar if you wanted. You'd have to moderator your ad posts to ensure people don't include their email address there though.
2. This is more about WordPress security, than specific to the theme. You can find a number of tutorials/tips on how to secure a WordPress site if you search Google. In terms of Payment information, this is all redirected via PayPal, so makes use of their security.
3. & 4. The theme and WordPress itself is well optimized for
SEO, but you can create a sitemap or use a sitemap plugin which will help with this as well.