I wouldn't touch this with a long stick, ebay has been a source of cracked/fake licenses and you could end up with a license that isn't legit and likely to be blocked/banned, as you mentioned. Whilst there is a possibility that this may be legit, the risk of such sales are high and the onus is on you to resolve any issues with the seller.
The authorised source for avast licenses are via avast.com or its authorised resellers.
http://www.avast.com/en-us/locate-dealer.
This license being sold through ebay Australia by an American seller would start my suspicious juices flowing; the 'No Returns' would be another. There are lots of Highlighted test purporting to legal/legit, etc. etc. but that is just text, the one thing missing is an authorised avast reseller information that can be checked out.
http://www.avast.com/en-us/locate-dealerPersonally it is safer and easier if you seek out an authorised reseller or purchase via the avast.com site. If this is from a legit avast reseller then other resellers should have the same deal.
Avast licenses once issued/sold are active from the date of purchase/sale, not when they are installed, so it is possible if this is a legit license that the clock may already be ticking.
