Hello, i have some question:
1)Its possible to sell hd/traffic from shared plans?
2)About resellers account... it's possible to use only one whmcs account to manage many hosgator reseller account?

I think those questions can be answered by HG themselves, have you asked them?

yes, i just asked them and they said can't answer question with third party sofwares -.-'

1. You can sell hd/traffic but your customers must be willing to purchase a hosting account without control panel.
2. A billing system can handle multiple reseller accounts/multiple servers from different providers.

1. You can sell hd/traffic but your customers must be willing to purchase a hosting account without control panel.Actually, no, reselling is not allowed on their shared hosting packages:

11.) Shared (non-reseller accounts) / Semidedicated Servers
Shared accounts may not resell web hosting to other people, if you wish to resell hosting you must use a reseller account. Semidedicated servers are not backed up. You must maintain your own backups.http://www.hostgator.com/tos.shtml

2)About resellers account... it's possible to use only one whmcs account to manage many hosgator reseller account?`
You should be able to do it, it's the same scenario than having many dedicated server to manage.

It should be, yes; Just a case of multi-servering with the WHM Remote Access Key