No it isn't legit, the longest legit license is for three years.
Generally they work to prevent the hack of the license and various checking, then the people using hacked licenses are offered the free version one day when they log on. I don't believe avast is actively seeking out sites that offer hacked copies of avast or cracked/keygen licenses.
So in this case people that are defrauded by these sales would end up having the license banned. They are the ones losing out. But it should be an offer that it too good to be true, no legit AV license is going to go out to 2050, as they say if the offer is to good to be true then frequently it is.
So those that know this is obviously not legit aren't going to have anything to complain about when the license gets banned.
EDIT: Typo
Always purchase through the vendors site or its list of authorised resellers.