Hi,
To show the text "incl. tax" or "excl. tax" on products you could potentially edit the view "product / listing_price" do a check on the user group and based on that display the correct text.
Regarding the checkout page where you need both prices displayed, the indicated method will not be optimal because for the resellers, no taxes are applied at all.
So in your case, you will have to handle that by code edition. Apply taxes for everybody and in the view "product / listing_price" change dynamically the value for the option "price_with_tax" based on the user group.
To know the user group, it can be found in the object "$user" thanks to that code:
$user = hikashop_loadUser(true);
And to change the option value:
$config = hikashop_config();
if($reseller){
$config->set('price_with_tax','0');
}else{
$config->set('price_with_tax','1');
}