What happens if the seller disappears or purchased item no more available


What happens if the extended purchased item disappear or the seller no more available?!

Will the buyer website and script still work or it will be disabled?!

How secure is the ownership of purchased scripts?!


Depends what you mean by extended purchase.

Envato cannot guarantee the lifespan of items (as is in the T&Cs) because authors retain ownership and can therefore remove items when they choose.

Even if an item is removed you should in theory still be ok to use it within the original license terms but there will no longer be access to re-download it or get updates.