Hey Mohan, thanks for your interest in buying at MOJO. Here's some answers to your questions.
1. Free updates. You get free upgrades to the latest version of a theme you've purchased. Updates to WordPress itself would be something you would do. Most theme updates are bug fixes or feature enhancements. For a quick example, you can scroll the bottom of this page to see the changelog of a theme - http://www.mojo-themes.com/item/everest-responsive-wordpress-theme/
2.Yes you get free support as long as the item is for sale on the marketplace and again, it's free.
3.Yes the seller is responsible but our support team is also here to provide general support as well as theme support. So really we offer a dual layer of support on items.
Hopefully that helps you out, let us know if you need anything else.